10299 Put PATH back in the environment, at least.

[exarkun]

Scope and purpose

CheckNewsfragmentScriptTests.setUp almost completely wipes the environment then the tests try to run git as a child process. This means the runCommand calls in the test methods have to find git with no PATH set which is unreliable at best.

Contributor Checklist:

• The associated ticket in Trac is here: https://twistedmatrix.com/trac/ticket/10299
• I ran tox -e lint to format my patch to meet the Twisted Coding Standard
• I have created a newsfragment in src/twisted/newsfragments/ (see: News files)
• The title of the PR starts with the associated Trac ticket number (without the # character).
• I have updated the automated tests and checked that all checks for the PR are green.
• I have submitted the associated Trac ticket for review by adding the word review to the keywords field in Trac, and putting a link to this PR in the comment; it shows up in https://twisted.reviews/ now.
• The merge commit will use the below format
  The first line is automatically generated by GitHub based on PR ID and branch name.
  The other lines generated by GitHub should be replaced.

Merge pull request #123 from twisted/4356-branch-name-with-trac-id

Author: exarkun
Reviewer: <github_username>, <github_username_if_more_reviewers>
Fixes: ticket:10299

Fix a spurious failure in twisted.python.test.test_release in some environments.

[exarkun]

The failure on "CI / 3.8-alldeps-withcov-posix (pull_request)" looks spurious to me.