This only works due to some assumptions about how the `ENVIRONMENT`
property is processed. Comments have been added to notify anyone
modifying the behavior about where to look.

Fixes: #22819

When appending or modifying to a variable that has not been modified
before, the iterator was not valid, but it was used to insert into the
map again. Instead, just use indexing into the map since we know it will
exist by the end of the function anyways.

Fixes: #22796

This property allows projects to modify environment variables at test
time rather than trying to guess what the state should be based on what
is present at configure time. Of particular interest is the ability to
use a `PATH` present at test time while adding entries known to be
necessary for the test itself.

There are multiple operations provided to modify variables, including:

  - setting and unsetting
  - appending and prepending as:
    - strings
    - path lists
    - CMake lists

Additionally, a `reset` action is provided to cancel any prior
modifications to that particular variable in the case of incremental
additions to the test property.

Run the `Utilities/Scripts/clang-format.bash` script to update
all our C++ code to a new style defined by `.clang-format`.
Use `clang-format` version 3.8.

* If you reached this commit for a line in `git blame`, re-run the blame
  operation starting at the parent of this commit to see older history
  for the content.

* See the parent commit for instructions to rebase a change across this
  style transition commit.