druntime integration tests: Fix musl detection with dash shell

[kinke]

As dash apparently doesn't understand the &> redirection, see ldc-developers/ldc#4970 (comment), causing IS_MUSL to be wrongly set on non-musl distros.

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[the-horo]

Note #21741

I think it's better to carry this musl detection in a common file like druntime/Makefile. This way we don't duplicate it in every test file that needs it. As a plus it also helps ldc by keeping its diff smaller since it uses a different mechanism for detecting musl. Changing it in one place (actually 0 places) is better than having to modify a bunch of different makefiles.

[kinke]

I'd have extracted it if a third file needs it. ;) - Unfortunately we cannot put it in existing common.mak, that's too late (as that include requires TESTS to be set up, and we filter those based on musl).

[the-horo]

Yeah, I got it wrong the first time by changing common.mak but moving it up in the dmd-druntime specific makefile is a win-win.

[kinke]

Note #21741

Oh damn, overlooked that at first because I thought you'd link to some post in the LDC repo. 🤦

[kinke]

Closing in favor of #21741.