mesa: Limit the devDoesNotDependOnLLVM test to Linux
mesa: Limit the devDoesNotDependOnLLVM test to Linux

The required modifications in the postInstall phase are only applied on
Linux and the test currently fails on Darwin:
https://github.com/NixOS/nixpkgs/runs/5344236204
> building '/nix/store/45s58pv9j6a19wr9izx49s6i0i4qshxs-mesa-dev-does-not-depend-on-llvm.drv'...
> error: output '/nix/store/czmszfcwdx87vx2wf80lhp3h9skqqcfs-mesa-dev-does-not-depend-on-llvm' is not allowed to refer to the following paths:
>          /nix/store/cwb5g57al7iizw456ah9rk49cxb47wi3-mesa-21.3.7-drivers

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