TestEnvironment.SetEnvironmentVariable should auto-invalidate MSBuild static caches

`TestEnvironment` is the shared test-isolation helper used across the MSBuild test suite. It already resets `ChangeWaves` and `BuildEnvironmentHelper` on dispose, so the next test starts clean.

However, when a test sets an environment variable that MSBuild reads through a cached static helper (the canonical case is `MSBUILDDISABLEFEATURESFROMVERSION`, which `ChangeWaves` reads once and caches), the new value is not observed by code under test in the same test method unless the test additionally calls:
```
ChangeWaves.ResetStateForTests();
BuildEnvironmentHelper.ResetInstance_ForUnitTestsOnly();
```

Make `TestEnvironment.SetEnvironmentVariable` invalidate the relevant static helpers automatically when the value is set. Then the manual `ChangeWaves.ResetStateForTests()` and `BuildEnvironmentHelper.ResetInstance_ForUnitTestsOnly()` calls inside test bodies (the sites listed above) can be removed.