Don't bother api.github.com on pull requests to avoid getting rate limited (#10709)

(cherry picked from commit c0389ef82f)

Co-authored-by: Michael Vines <mvines@gmail.com>

docs/set-solana-release-tag.sh

@@ -5,12 +5,16 @@ cd "$(dirname "$0")"
 
 if [[ -n $CI_TAG ]]; then
   LATEST_SOLANA_RELEASE_VERSION=$CI_TAG
-else
+elif [[ -z $CI_PULL_REQUEST ]]; then
   LATEST_SOLANA_RELEASE_VERSION=$(\
     curl -sSfL https://api.github.com/repos/solana-labs/solana/releases/latest \
     | grep -m 1 tag_name \
     | sed -ne 's/^ *"tag_name": "\([^"]*\)",$/\1/p' \
   )
+else
+  # Don't bother the `api.github.com` on pull requests to avoid getting rate
+  # limited
+  LATEST_SOLANA_RELEASE_VERSION=unknown-version
 fi
 
 set -x