GHA main: Add Alpine Linux job

[kinke]

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[dlang-bot]

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[kinke]

@JohanEngelen: Looks like there's no exception stack trace support for musl here upstream. I guess we'd need to upstream some druntime changes in ldc-developers/ldc#4639 and ldc-developers/ldc#4691?

[JohanEngelen]

@JohanEngelen: Looks like there's no exception stack trace support for musl here upstream. I guess we'd need to upstream some druntime changes in ldc-developers/ldc#4639 and ldc-developers/ldc#4691?

yes indeed!

[kinke]

@JohanEngelen: Okay, I've merged your two PRs to a single commit suitable for upstream here, as 2nd commit; the 3rd commit shows that it works, allowing to re-enable most hacked-out tests. Please extract it to its own PR and make sure it gets merged, as prerequisite of this PR. [It's your work after all, and you probably remember most details that could come up during a review.]