Temporarily disable musl builds until CI upgraded

[nathaniel-bennett]

Temporarily disables musl checks (which is no longer working due to rustc requiring v1.2+) until the musl CI images can be updated.

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[nathaniel-bennett]

Nightly freebsd-15 seems to be broken on libc-0.2, but fixing that would be outside the scope of this PR.

[nathaniel-bennett]

(temporarily fixes #3613)

[JohnTitor]

Thanks! Now CI works fine, closing.