ci: Re-enable the musl build on windows-gnu

[tgross35]

ci: test-libm

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[tgross35]

The build+run works but shows a bunch of failures like:

        thread 'musl_edge_case_expf' (1728) panicked at libm-test\tests\compare_built_musl.rs:26:50:
        called `Result::unwrap()` on an `Err` value:
          input:    (0.0,)
          as hex:   (+0x0p+0,)
          as bits:  (0x00000000,)
          expected: 0.0                 +0x0p+0   0x00000000
          actual:   1.0                 +0x1.000000p+0   0x3f800000

        thread 'musl_edge_case_ceil' (1964) panicked at libm-test\tests\compare_built_musl.rs:26:50:
        called `Result::unwrap()` on an `Err` value:
          input:    (1.0000000000000002,)
          as hex:   (+0x1.0000000000001p+0,)
          as bits:  (0x3ff0000000000001,)
          expected: 1.0                 +0x1.0000000000000p+0   0x3ff0000000000000
          actual:   2.0                 +0x1.0000000000000p+1   0x4000000000000000

I don't think it's worth pursuing.