Temporarily (hopefully) pin opencl-mode

This pins to the version Doom pinned to up until recently.

fetch-overrides.nix

@@ -53,5 +53,10 @@
     # making this a non-issue for Doom (since there have been no significant changes to git-commit).
     # CI should fail again the next time Doom updates magit, so this won't go stale.
     git-commit = "ea0f07e54967197ac0b072a69ba314314a4080c1";
+
+    # Temporarily pin opencl-mode. It renamed to opencl-c-mode, which triggers a build failure:
+    #
+    # error("%s" "Single file opencl-c-mode.el does not match package name opencl-mode")
+    opencl-mode = "15091eff92c33ee0d1ece40eb99299ef79fee92d";
   };
 }