druntime-test: Detect musl more robustly

[the-horo]

Try to detect musl on more systems than just Alpine.

This also fixes an issue in the Github runners where the default shell is dash which doesn't understand the &> operator leading to the musl-excluded tests to also be skipped on glibc linux.

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Testing this PR locally

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[the-horo]

Spotted this in the ldc CI: ldc-developers/ldc#4969 (comment)