tests: Skip supported-crate-types test on musl hosts
This test depends on the target-specific default of crt-static for musl targets. However, running the testsuite on a musl host requires setting `crt-static` to `false`, as it wouldn't otherwise be possible to build rustc. This in turn will enable `-Ctarget-feature=-crt-static` for all tests, mismatching the expected `+crt-static` for the musl target tested in this testcase.
Since this test specifically tests the default value of `crt-static` for the musl target, ignoring it entirely makes more sense than manually setting `-Ctarget-feature=+crt-static` here, but both would be valid approaches.
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