CI: remove nim from the ubuntu rolling install list

We've never used it for anything, it was originally added for #3776 but
that never got finished so it's just a waste.

This also prevents successful regeneration of the build image, because
nim is not available for Ubuntu rolling. It's available in 20.04 and
22.10, but vanished in between for reasons best known to Ubuntu.

ci/ciimage/ubuntu-rolling/install.sh

@@ -15,7 +15,7 @@ pkgs=(
   libomp-dev
   llvm lcov
   dub ldc
-  mingw-w64 mingw-w64-tools nim
+  mingw-w64 mingw-w64-tools
   libclang-dev
   libgcrypt20-dev
   libgpgme-dev