[Github] Fix security issues in libcxx-run-benchmarks.yml workflow (#158467)

There was one action dependency that was not hash pinned and this
workflow also allowed code injection as the input might not be properly
escaped when dumped into the run script.

Author: boomanaiden154

.github/workflows/libcxx-run-benchmarks.yml

@@ -33,12 +33,14 @@ jobs:
 
     runs-on: llvm-premerge-libcxx-next-runners # TODO: This should run on a dedicated set of machines
     steps:
-      - uses: actions/setup-python@v6
+      - uses: actions/setup-python@e797f83bcb11b83ae66e0230d6156d7c80228e7c # v6.0.0
         with:
           python-version: '3.10'
 
       - name: Extract information from the PR
         id: vars
+        env:
+          COMMENT_BODY: ${{ github.event.comment.body }}
         run: |
           python3 -m venv .venv
           source .venv/bin/activate
@@ -51,7 +53,7 @@ jobs:
           print(f"pr_base={pr.base.sha}")
           print(f"pr_head={pr.head.sha}")
           EOF
-          BENCHMARKS=$(echo "${{ github.event.comment.body }}" | sed -nE 's/\/libcxx-bot benchmark (.+)/\1/p')
+          BENCHMARKS=$(echo "$COMMENT_BODY" | sed -nE 's/\/libcxx-bot benchmark (.+)/\1/p')
           echo "benchmarks=${BENCHMARKS}" >> ${GITHUB_OUTPUT}
 
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0