Make platform checks throw BuildError like other failures

[gustavderdrache]

Motivation

Some complex derivations can depend on multiple systems. In cases where these aren't already cached (or locally buildable), Nix throws an error indicating that a derivation cannot be built. However, this does not behave the same with respect to other build errors. This PR changes the error to throw BuildError, allowing Nix to report the dependency tree that led to the build error, if any.

In the examples below, needs-* depends on a derivation that fails for the various scenarios I've tested locally.

Mismatched system

Before:

$ nix build .#needs-system
error: a 'bogus' with features {} is required to build '/nix/store/1aj7zljmh3zwff9z0pcan940kghql87c-bad-system.drv', but I am a 'x86_64-linux' with features {benchmark, big-parallel, kvm, nixos-test, uid-range}

After:

$ nix build .#needs-system
error: Cannot build '/nix/store/1aj7zljmh3zwff9z0pcan940kghql87c-bad-system.drv'.
       Reason: required system or feature not available
       Required system: 'bogus' with features {}
       Current system: 'x86_64-linux' with features {benchmark, big-parallel, kvm, nixos-test, uid-range}
error: Cannot build '/nix/store/xw31yf7yldxk163858vb8n51j9zbwnz7-needs-system.drv'.
       Reason: 1 dependency failed.
       Output paths:
         /nix/store/9qralxw4zvb5s9cii2lv04l66nygr95r-needs-system

Missing required feature

Before:

$ nix build .#needs-feature
error: a 'x86_64-linux' with features {bogus} is required to build '/nix/store/cny1i79kcnj8cbzm17bzmnn6f74jl0rr-bad-feature.drv', but I am a 'x86_64-linux' with features {benchmark, big-parallel, kvm, nixos-test, uid-range}

After:

$ nix build .#needs-feature
error: Cannot build '/nix/store/cny1i79kcnj8cbzm17bzmnn6f74jl0rr-bad-feature.drv'.
       Reason: required system or feature not available
       Required system: 'x86_64-linux' with features {bogus}
       Current system: 'x86_64-linux' with features {benchmark, big-parallel, kvm, nixos-test, uid-range}
error: Cannot build '/nix/store/ki2sn6hb4mkv0sg6c676lllmzs2alzxd-needs-feature.drv'.
       Reason: 1 dependency failed.
       Output paths:
         /nix/store/q8mp4h0s6g0qq6z72kwidlp5a7m7a9qx-needs-feature

Rosetta not installed

(Note that I don't have a Mac to test on, but I've used --system passed to sudo nix build to generate this error.)

Before:

$ nix build .#mac-system
error: run `/usr/sbin/softwareupdate --install-rosetta` to enable your aarch64-darwin to run programs for x86_64-darwin

After:

$ nix build .#mac-system
error: Cannot build '/nix/store/2iq7bj3zgsggbn1q6vdn79fk1wiksldy-mac-system.drv'.
       Reason: required system or feature not available
       Required system: 'x86_64-darwin' with features {}
       Current system: 'aarch64-darwin' with features {benchmark, big-parallel, kvm, nixos-test, uid-range}
       Note: run `/usr/sbin/softwareupdate --install-rosetta` to run programs for x86_64-darwin

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