Add workaround for buggy rubygems in Ruby 3.4.0-preview2

Got bitten by rubygems/rubygems#8108 again.

.github/workflows/test-memory-leaks.yaml

@@ -12,6 +12,7 @@ jobs:
           bundler: latest
           cache-version: v1 # bump this to invalidate cache
       - run: sudo apt-get update && (sudo apt-get install -y valgrind || sleep 5 && sudo apt-get install -y valgrind) && valgrind --version
+      - run: gem update --system 3.5.23 # TODO: This is a workaround for a buggy rubygems in 3.4.0-preview2; remove once stable version 3.4 is out
       - run: bundle exec rake compile spec:profiling:memcheck
   test-asan:
     # Temporarily ruby-asan builds changes