Fix potential vulnerability in CI workflow

[joewallwork]

Supersedes #1.

As noted in geoschem#86, without the edits to the cloud benchmarking workflow in this PR, the zizmor GitHub Actions static analysis tool reports:

error[template-injection]: code injection via template expansion
  --> /home/joe/software/GCClassic/.github/workflows/cloud-benchmarking-workflow.yml:45:9
   |
45 |         - name: Reset Initial Variables for pull request
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this step
46 | /         run: |
47 | |           echo "GITHUB_SHA_SHORT=`echo ${{ github.event.pull_request.head.sha }} | cut -c1-7`" >> $GITHUB_ENV
48 | |           echo "COMMIT_NAME=`echo ${{ github.event.pull_request.head.sha }} | cut -c1-7`" >> $GITHUB_ENV
   | |________________________________________________________________________________________________________^ github.event.pull_request.head.sha may expand into attacker-controllable code
   |
   = note: audit confidence → High

error[template-injection]: code injection via template expansion
  --> /home/joe/software/GCClassic/.github/workflows/cloud-benchmarking-workflow.yml:45:9
   |
45 |         - name: Reset Initial Variables for pull request
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this step
46 | /         run: |
47 | |           echo "GITHUB_SHA_SHORT=`echo ${{ github.event.pull_request.head.sha }} | cut -c1-7`" >> $GITHUB_ENV
48 | |           echo "COMMIT_NAME=`echo ${{ github.event.pull_request.head.sha }} | cut -c1-7`" >> $GITHUB_ENV
   | |________________________________________________________________________________________________________^ github.event.pull_request.head.sha may expand into attacker-controllable code
   |
   = note: audit confidence → High