Allow cmake toolchain env

[justsmth]

Issues:

Resolves #376

Description of changes:

• cargo-xwin is a build tool for "Cross compil[ing a] Cargo project to Windows msvc target with ease".
• cargo-xwin sets a CMAKE_TOOLCHAIN_FILE_*_pc_windows_msvc environment variable to provide CMake guidance on the compiler setup. We should avoid overriding the configuration it provides.

Testing:

• Adds a CI job to verify that we can build for the *_pc_windows_msvc targets from an Ubuntu host.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.67%. Comparing base (c358484) to head (ca9eb35).
Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #431      +/-   ##
==========================================
- Coverage   95.80%   92.67%   -3.13%     
==========================================
  Files          61       61              
  Lines        8143     8174      +31     
  Branches        0     8174    +8174     
==========================================
- Hits         7801     7575     -226     
- Misses        342      357      +15     
- Partials        0      242     +242     

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