Revert "buildRustPackage: fix cargoBuildFlags"

This reverts commit deb78151a9.

Mixing up two distinct phases of a derivation's build is not a good idea. See
also https://github.com/NixOS/nixpkgs/pull/91689#issuecomment-657813954.

pkgs/build-support/rust/default.nix

@@ -199,7 +199,7 @@ stdenv.mkDerivation (args // {
       -executable ! \( -regex ".*\.\(so.[0-9.]+\|so\|a\|dylib\)" \))
   '';
 
-  installCheckPhase = args.checkPhase or (let
+  checkPhase = args.checkPhase or (let
     argstr = "${stdenv.lib.optionalString (checkType == "release") "--release"} --target ${rustTarget} --frozen";
   in ''
     ${stdenv.lib.optionalString (buildAndTestSubdir != null) "pushd ${buildAndTestSubdir}"}

pkgs/development/tools/rust/rustup/default.nix

@@ -39,9 +39,7 @@ rustPlatform.buildRustPackage rec {
     )
   ];
 
-  # Disable tests until they can be run with --features no-self-update
-  doCheck = false;
-  #doCheck = !stdenv.isAarch64 && !stdenv.isDarwin;
+  doCheck = !stdenv.isAarch64 && !stdenv.isDarwin;
 
   postInstall = ''
     pushd $out/bin