Disable aws-cli v2 default checksums for GCS S3-compat compatibility

aws-cli v2 started sending x-amz-checksum-* headers by default; GCS's
XML API rejects them with SignatureDoesNotMatch on UploadPart.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: akariv

.github/workflows/backup.yml

@@ -28,6 +28,9 @@ jobs:
           AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
           AWS_REGION: ${{ secrets.AWS_REGION }}
           S3_ENDPOINT_URL: ${{ secrets.S3_ENDPOINT_URL }}
+          # GCS S3-compat rejects aws-cli v2's default x-amz-checksum-* headers
+          AWS_REQUEST_CHECKSUM_CALCULATION: when_required
+          AWS_RESPONSE_CHECKSUM_VALIDATION: when_required
         run: |
           set -euo pipefail
           FILENAME="backup_$(date -u +%Y%m%d_%H%M%SZ).pgdump"