fix: use FETCH_HEAD instead of branch name in git checkout

git checkout -- <name> treats the argument as a pathspec, not a branch
name, causing "pathspec did not match" errors. Using FETCH_HEAD avoids
passing any user-controlled input to the checkout command entirely.

Author: lorenarosati

.github/scripts/run-benchmarks.sh

@@ -85,7 +85,7 @@ echo "Parsed filter: ${FILTER:-<none>}"
 #    "changes" baseline files are preserved across the branch switch.
 # ---------------------------------------------------------------------------
 git fetch origin -- "$BASE_REF"
-git checkout -- "$BASE_REF"
+git checkout FETCH_HEAD
 (cd benchmarks && cargo bench --locked --bench workload_bench -- --save-baseline base "$FILTER")
 
 # ---------------------------------------------------------------------------