Use absolute path to /usr/bin/xattr instead of pulling whatever is in PATH

On macOS systems with Homebrew installed, the latter will install its
own copy of `xattr` in `/opt/homebrew/bin/xattr` which will often
take precedence over the system `xattr` at `/usr/bin/xattr`, and does
not support the `-r` flag to act recursively over a directory.

This commit changes the invocation of `xattr` to use the absolute path
to the system version of `xattr` at `/usr/bin/xattr`.

lib/GHCup/Utils.hs

@@ -1081,7 +1081,7 @@ darwinNotarization :: (MonadReader env m, HasDirs env, MonadIO m)
                    -> FilePath
                    -> m (Either ProcessError ())
 darwinNotarization Darwin path = exec
-  "xattr"
+  "/usr/bin/xattr"
   ["-r", "-d", "com.apple.quarantine", path]
   Nothing
   Nothing