buildNoTests: Restore intent

The thing we wanted to test was that building Nix without building or
running tests, and without depending on libraries only needed by tests,
works.

But since 6c8f4ef350, we can also install
unit tests, and during the conversion to using `package.nix` this
started happening more often (they go to a separate output though, so
this should be fine).

This adds more `... = false` to restore the original intent: don't run
unit test or functional tests, and don't install unit tests.

flake.nix

@@ -234,11 +234,11 @@
         buildNoGc = forAllSystems (system: self.packages.${system}.nix.overrideAttrs (a: { configureFlags = (a.configureFlags or []) ++ ["--enable-gc=no"];}));
 
         buildNoTests = forAllSystems (system:
-          self.packages.${system}.nix.overrideAttrs (a: {
-            doCheck =
-              assert ! a?dontCheck;
-              false;
-          })
+          self.packages.${system}.nix.override {
+            doCheck = false;
+            doInstallCheck = false;
+            installUnitTests = false;
+          }
         );
 
         # Perl bindings for various platforms.