7417: Fix bundler/inline warning: method redefined; discarding old root r=deivid-rodriguez a=fatkodima

Closes #6167

Co-authored-by: fatkodima <fatkodima123@gmail.com>

7416: Don't use insecure temporary directory as home directory r=deivid-rodriguez a=fatkodima

Closes #6501 

Co-authored-by: fatkodima <fatkodima123@gmail.com>

7427: Pass `env` as a parameter to `sys_exec` r=deivid-rodriguez a=deivid-rodriguez

### What was the end-user problem that led to this PR?

The problem was that Windows has trouble with commands of the form `BUNDLE_FROZEN=true bundle install`.

### What was your diagnosis of the problem?

My diagnosis was that we should use `Open3.popen3` explicit `env` argument instead of passing a command line that has environment variables in the front.

### What is your fix for the problem, implemented in this PR?

My fix is to do just that. This is a follow up to #7011, but specifically for the `ruby` test helper. I expect this to fix some more Windows failures.

Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>

7426: Revert "I don't think we need this now" r=deivid-rodriguez a=deivid-rodriguez

This reverts commit 36816866581147ba7bd491a2a658ac35a2f03afe.

### What was the end-user problem that led to this PR?

The problem was that in #7401, after I got specs passing against a non-silenced by default UI, I started undoing workarounds that I thought were due to the silent by default UI. 

### What was your diagnosis of the problem?

My diagnosis was that in the case I'm reverting here this is not actually fixed, [as the Azure Pipelines logs confirm](https://dev.azure.com/bundler/bundler/_build/results?buildId=2662&view=ms.vss-test-web.build-test-results-tab&runId=1003360&resultId=101526&paneView=debug).

### What is your fix for the problem, implemented in this PR?

My fix is to unrevert the specific commit that I shouldn't have reverted.


Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>

This reverts commit 36816866581147ba7bd491a2a658ac35a2f03afe.

7425: Properly avoid readline crashes when running specs on Windows r=deivid-rodriguez a=deivid-rodriguez

### What was the end-user problem that led to this PR?

The problem was that specs cannot be run on windows unless you add the ugly workaround currently included in the azure pipelines config.

### What was your diagnosis of the problem?

My diagnosis was that the `tmp/tmpdir` and `tmp/home` folder (which act as `ENV["HOME"]` and `ENV["TMPDIR"]` for our specs) are emptied in an `around(:each)` hook, but there's some stuff going before that and after `ENV["HOME"]` and `ENV["TMPDIR"]` are set (on a `before(:suite)` hook) that might require the folders they point to to exist.

This is the case of rb-readline, used on Windows:

https://github.com/ConnorAtherton/rb-readline/blob/9fba246073f78831b7c7129c76cc07d8476a8892/lib/rbreadline.rb#L1096

or any bundler's code that uses `Bundler.user_home`:

https://github.com/bundler/bundler/blob/f83412ae8cda9c933b8cf33ec2abfb78a408daab/lib/bundler.rb#L227

For example, running `rm -rf tmp/ && bin/rspec ./spec/other/major_deprecation_spec.rb:368` prints a warning about this that it's fixed by this PR:

```
$ rm -rf tmp/ && bin/rspec ./spec/other/major_deprecation_spec.rb:368 

Randomized with seed 1955

(... stuff ...)

`/home/deivid/Code/bundler/tmp/1/home` is not a directory.
Bundler will use `/tmp/bundler/home/deivid' as your home directory temporarily.
.

Retried examples: 0


Finished in 5.26 seconds (files took 0.17927 seconds to load)
1 example, 0 failures

Randomized with seed 1955
```

### What is your fix for the problem, implemented in this PR?

My fix is to create the folders as soon as the environment variables are set.

### Why did you choose this fix out of the possible options?

I chose this fix because it's more general that a workaround living on a specific CI config.

Fixes #6902.

Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>

7424: Drop unnecessary `tempfile` require r=deivid-rodriguez a=deivid-rodriguez

### What was the end-user problem that led to this PR?

The `tempfile` dependency was dropped in [1], but the require was left in there. This could cause gem conflicts because `tempfile` requires `tmpdir` which requires `fileutils`, which loads the default gem instead of our namespaced version, and that could cause fileutils version mismatches and code overriding warnings.

[1]: https://github.com/bundler/bundler/commit/4a37d66f3f222739178d798b30fb135f2429fe45

### What is your fix for the problem, implemented in this PR?

My fix is to drop the unnecessary require.


Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>

This also avoids the following warning when running specs on any
platform:

```
`/home/deivid/Code/bundler/tmp/1/home` is not a directory.
Bundler will use `/tmp/bundler/home/deivid' as your home directory temporarily
```