6743: Support file structure of ruby core repository. r=hsbt a=hsbt

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

In the ruby core repository, I put bundler executable and bundler libraries under the `bin` and `lib` directories. It breaks the current behavior.

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

Support the structure of ruby core repository.



Co-authored-by: SHIBATA Hiroshi <hsbt@ruby-lang.org>

6714: Spec reset state and refactorings r=deivid-rodriguez a=deivid-rodriguez

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

The problem was that while working on #6713, I noticed several issues like order dependent failures, situations hard to debug, or specs doing too much and taking longer than they should.
 
### What was your diagnosis of the problem?

My diagnosis was that:

* Sometimes specs leak settings and env variable modifications.
* When a hang happens inside a subprocess, it's hard to debug because logging is not printed out anywhere.
* Some specs create unnecessary gemfiles and run `bundle install` multiple times unnecessarily.

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

My fix for state leaks is to reset state after each spec, for the hangs is to reorder some lines in the `sys_exec` helper (https://github.com/bundler/bundler/commit/b305a5b2524a6457b05a9d39e9526f75c98a0752), and for the unnecessary operations, to refactor the specs to avoid them.

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

I chose this fix because it seems like the best way to alleviate the issues found.


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

Since we no longer test against rubygems versions that old.

  They are replaced build binary on ruby core repository.

6742: Use helper method instead of hard-code path. r=colby-swandale a=hsbt

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

In ruby core repository, We need to replace executable file like `ruby`, `rake`, `gem`. 

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

This pull request makes hard-coded executable file to the helper methods.



Co-authored-by: SHIBATA Hiroshi <hsbt@ruby-lang.org>