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 :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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