buildRustCrate: fix target config environment variables on 32-bit ARM

[lopsided98]

Motivation for this change

Followup to #99050, fixing issues I found while trying to cross-compile some other packages with crate2nix.

Because TARGET wasn't set correctly, the autocfg crate was unable to run its compile tests, and downstream crates didn't get features enabled correctly.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

cc @Ericson2314 @danieldk @kolloch @andir @symphorien

[andir]

This is looking good! Can you also add a minimal cross compilation test for this? I'd hate to have this fail silently due to other changes/refactorings. It is a corner-case after all.

[andir]

@GrahamcOfBorg build buildRustCrateTests

[lopsided98]

I made buildRustCrateTests succeed when cross-compiling.

@ofborg build buildRustCrateTests pkgsCross.armv7l-hf-multiplatform.buildRustCrateTests

[Ericson2314]

Happy to merge once tests pass! (Or without if they take forever because things are not yet cached.)

[lopsided98]

The test failure looks like a transient ofborg issue. I'll try to run it again.

@ofborg build buildRustCrateTests pkgsCross.armv7l-hf-multiplatform.buildRustCrateTests

[lopsided98]

@ofborg build pkgsCross.armv7l-hf-multiplatform.buildRustCrateTests

[lopsided98]

@ofborg build buildRustCrateTests

[Ericson2314]

OK screw it, merging.