Context

MSBuild has a command-line switch that tells it to output all environment variables. C++ used that fairly regularly. This PR automatically opts out of the environment variable logging change in #7484 for the duration of the one build.

Changes Made

Made the Trait settable via environment variable or with the file logger parameter. Since the parameter needs to be applied first, this means the Trait has to be set after the start of the build, so it is no longer read-only. In tests, Traits.Instance is reset every time a variable is read, so setting it at the start of the test does not work. For that reason, I had to make it a static variable on Traits instead of Traits.Instance.

Of importance, the Trait is reset (to the environment variable's value) at logger shutdown for any BaseConsoleLogger. There is theoretically a race condition here, since the Trait is set at logger initialization (if the parameter is set) and used for the BuildStarted event. If there are two builds executing concurrently in the entrypoint node, and loggers are initialized for both, then one finishes before the other has gotten to BuildStarted, the latter will not log the environment. On the other hand, that doesn't affect whether your build succeeds, and the resolution is to just run the build again, and it will likely succeed.

Testing

It passed unit tests. I also created an experimental insertion, and it passed the C++ test.