Auto merge of #25728 - rib:wip/rib/windows-build-fixes, r=jdm

Wip/rib/windows build fixes

These were minor changes I needed to make to build Servo on Windows based on following the existing instructions. In particular:

Even though I have C:\Python27 with python.exe in my PATH the build failed at some point when it failed to find python27.exe and I found that explicitly setting the PYTHON2 environment variable resolved this failure.

The end of the build failed to package MSVC redistributable DLLs and that was simply because I have a version (142) installed that wasn't checked for by build_commands.py.

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<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix builds on Windows if you have version 142 MSVC redistributable DLLs and if you don't have a python2.7exe or python27.exe in your PATH.

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- [x] These changes do not require tests because they don't change any servo code

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