Add retry logic to stdgems.org curl requests in CI

The curl requests to stdgems.org occasionally fail with connection
errors, causing spurious CI failures. Add curl's built-in retry
flags to handle transient network issues with exponential backoff.

example:
https://github.com/ruby/ruby/actions/runs/23825769292/job/69448266927

Author: k0kubun

.github/workflows/bundled_gems.yml

@@ -56,7 +56,7 @@ jobs:
           mkdir -p .downloaded-cache
           for data in bundled_gems.json default_gems.json; do
             ln -s .downloaded-cache/$data .
-            curl -O -R -z ./$data https://stdgems.org/$data
+            curl --retry 5 --retry-connrefused --retry-delay 2 --retry-max-time 60 -O -R -z ./$data https://stdgems.org/$data
           done
 
       - name: Update bundled gems list

.github/workflows/default_gems_list.yml

@@ -43,7 +43,7 @@ jobs:
           data=default_gems.json
           mkdir -p .downloaded-cache
           ln -s .downloaded-cache/$data .
-          curl -O -R -z ./$data https://stdgems.org/$data
+          curl --retry 5 --retry-connrefused --retry-delay 2 --retry-max-time 60 -O -R -z ./$data https://stdgems.org/$data
         if: ${{ steps.gems.outcome == 'success' }}
 
       - name: Make default gems list