ci: Use Ruby 3.4 instead of the recently released 4.0 for now

Ruby 4.0 is broken upstream in Vim. Once that's fixed, we can remove
this.

Author: ychin

.github/workflows/macvim-buildtest.yaml

@@ -137,7 +137,7 @@ jobs:
           # be installed on runners. Since we use stable ABI, the exact version
           # on CI does not matter.
 
-          brew install --quiet ruby
+          brew install --quiet ruby@3.4 # Ruby 4.0 is broken in Vim. Use 3.4 until that's fixed upstream.
           brew install --quiet lua
 
           if [[ -d $(brew --prefix)/Cellar/perl ]]; then