Fix meson build on macOS in sandbox

[YorikSar]

Motivation

Workaround at src/libstore/meson.build#L429-L434 by @Ericson2314 from #11302 erroneously used macos instead of darwin to distinguish macOS, while meson docs list only darwin: https://mesonbuild.com/Reference-tables.html#operating-system-names.

Context

Original thread: #2503 (comment)

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[Ericson2314]

Thank you so much!

[yannham]

Nitpick: this isn't entirely true. macos is listed in subsytems in https://mesonbuild.com/Reference-tables.html#subsystem-names-since-120 which says that it should cover OsX, while darwin is a superset including iOS. This is why I thought this wasn't the culprit initially, but if it solves the issue 🤷‍♂️ in anycase darwin is just more generic, so I don't think there's any particular risk here.

[YorikSar]

@yannham The title there differs though: the first table lists system names, the second - subsystem names. It doesn't point to the function at the table, but https://mesonbuild.com/Cross-compilation.html#machine-entries points to .subsystem() function that I assume is supposed to return those strings.

[Ericson2314]

@YorikSar Yeah I think it will accept macos as a configuration input, but the result of host_machine.system() will still be darwin.

[yannham]

Ah, I see. Of course it can't be both darwin and macos so what I said doesn't make sense 🤦‍♂️ still, the documentation could be clearer, as currently it doesn't mention subsystem() at all (while the section on system says explicitly that those are the result of calling the method .system()). Anyway, thanks for the fix and the clarification!