Avoid overwriting env variable

See https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/store-information-in-variables#default-environment-variables

.github/workflows/haskell-actions.yml

@@ -32,14 +32,14 @@ jobs:
       - name: Extract New-Versions git trailer from Renovate
         if: ${{ github.event_name == 'pull_request' }}
         env:
-          GITHUB_SHA: "{{ github.event.pull_request.head.sha }}"
+          HEAD_SHA: "{{ github.event.pull_request.head.sha }}"
         run: |
           if [ ! -f cabal.project ]
             then echo 'packages: .' > cabal.project
           fi
-          echo $GITHUB_SHA
-          git show $GITHUB_SHA
-          for constraint in $(git log "--format=%(trailers:key=New-Versions,valueonly=true)" ${GITHUB_SHA}^1..${GITHUB_SHA}^2)
+          echo $HEAD_SHA
+          git show $HEAD_SHA
+          for constraint in $(git log "--format=%(trailers:key=New-Versions,valueonly=true)" ${HEAD_SHA} -1
             do echo "constraints: $constraint" >> cabal.project
           done
           cat cabal.project