git: use finalAttrs for doInstallCheck config

For easier overriding with `git.override` or `git.overrideAttrs`,
determine whether configuration related to running the install checks is
present based on whether the install checks are actually being run, not
on the function argument which may not be the same.

pkgs/by-name/gi/git/package.nix

@@ -157,7 +157,7 @@ stdenv.mkDerivation (finalAttrs: {
     substituteInPlace contrib/credential/libsecret/Makefile \
         --replace-fail 'pkg-config' "$PKG_CONFIG"
   ''
-  + lib.optionalString doInstallCheck ''
+  + lib.optionalString finalAttrs.doInstallCheck ''
     # ensure we are using the correct shell when executing the test scripts
     patchShebangs t/*.sh
   ''