Poison the build on the test derivation

Make sure that we're not accidentally rebuilding Nix here as it's just
wasteful and awful for CI times.

flake.nix

@@ -280,9 +280,13 @@
         enableParallelBuilding = true;
 
         configureFlags = testConfigureFlags; # otherwise configure fails
-        dontBuild = true;
         doInstallCheck = true;
 
+        buildPhase = ''
+          # Remove the source files to make sure that we're not accidentally rebuilding Nix
+          rm src/**/*.cc
+        '';
+
         installPhase = ''
           mkdir -p $out
         '';