chore: get and use commit of branch we are trying to build (supabase#1614)

Author: samrose

.github/workflows/ami-release-nix-single.yml

@@ -28,6 +28,11 @@ jobs:
         with:
           ref: ${{ github.event.inputs.branch }}
 
+      - name: Get current branch SHA
+        id: get_sha
+        run: |
+          echo "sha=$(git rev-parse HEAD)" >> $GITHUB_OUTPUT
+
       - uses: DeterminateSystems/nix-installer-action@main
 
       - name: Set PostgreSQL version environment variable
@@ -46,15 +51,15 @@ jobs:
           POSTGRES_MAJOR_VERSION: ${{ env.POSTGRES_MAJOR_VERSION }}
         run: |
           packer init amazon-arm64-nix.pkr.hcl
-          GIT_SHA=${{github.sha}}
+          GIT_SHA=${{ steps.get_sha.outputs.sha }}
           packer build -var "git-head-version=${GIT_SHA}" -var "packer-execution-id=${GITHUB_RUN_ID}"  -var-file="development-arm.vars.pkr.hcl" -var-file="common-nix.vars.pkr.hcl" -var "ansible_arguments=-e postgresql_major=${POSTGRES_MAJOR_VERSION}"  amazon-arm64-nix.pkr.hcl
 
       - name: Build AMI stage 2
         env:
           POSTGRES_MAJOR_VERSION: ${{ env.POSTGRES_MAJOR_VERSION }}
         run: |
           packer init stage2-nix-psql.pkr.hcl
-          GIT_SHA=${{github.sha}}
+          GIT_SHA=${{ steps.get_sha.outputs.sha }}
           POSTGRES_MAJOR_VERSION=${{ env.POSTGRES_MAJOR_VERSION }}
           packer build -var "git_sha=${GIT_SHA}" -var "git-head-version=${GIT_SHA}" -var "packer-execution-id=${GITHUB_RUN_ID}" -var "postgres_major_version=${POSTGRES_MAJOR_VERSION}" -var-file="development-arm.vars.pkr.hcl" -var-file="common-nix.vars.pkr.hcl" stage2-nix-psql.pkr.hcl
 
@@ -66,7 +71,7 @@ jobs:
 
       - name: Create nix flake revision tarball
         run: |
-          GIT_SHA=${{github.sha}}
+          GIT_SHA=${{ steps.get_sha.outputs.sha }}
           MAJOR_VERSION=${{ env.POSTGRES_MAJOR_VERSION }}
 
           mkdir -p "/tmp/pg_upgrade_bin/${MAJOR_VERSION}"
@@ -116,7 +121,7 @@ jobs:
         with:
           name: ${{ steps.process_release_version.outputs.version }}
           tag_name: ${{ steps.process_release_version.outputs.version }}
-          target_commitish: ${{github.sha}}
+          target_commitish: ${{ steps.get_sha.outputs.sha }}
 
       - name: Slack Notification on Failure
         if: ${{ failure() }}