rustPlatform.cargoSetupHook: fix platform check (#260068)

Cargo will never need to link for the target platform — that'd be for
the package being built to do at runtime. Cargo should know about the
build and host linkers.

This fixes e.g. pkgsCross.musl64.fd from x86_64-linux.

Fixes: 67a4f828b46c ("rust: hooks: fix cross compilation")

pkgs/build-support/rust/hooks/default.nix

···
         cargoConfig = ''
           [target."${rust.toRustTarget stdenv.buildPlatform}"]
           "linker" = "${rust.envVars.ccForBuild}"
-          ${lib.optionalString (stdenv.hostPlatform.config != stdenv.targetPlatform.config) ''
-            [target."${rust.toRustTarget stdenv.targetPlatform}"]
-            "linker" = "${rust.envVars.ccForTarget}"
+          ${lib.optionalString (stdenv.buildPlatform.config != stdenv.hostPlatform.config) ''
+            [target."${rust.toRustTarget stdenv.hostPlatform}"]
+            "linker" = "${rust.envVars.ccForHost}"
           ''}
           "rustflags" = [ "-C", "target-feature=${if stdenv.hostPlatform.isStatic then "+" else "-"}crt-static" ]
         '';