win, build: add arbitrary MSBuild flag option and binlog option
This commit adds a 'verbose' flag to vcbuild.bat that turns on outputs useful for build debugging, including MSBuild BinLogging and DEBUG_HELPER logging.
It also adds a 'setlocal' call so internal environment variables no longer leak out to the calling shell.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
[Edit: Note new commit message (cf0681c
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