fix: use /tmp for TMPDIR instead of PWD/tmp

/tmp should always be writable in containers, whereas PWD/tmp
might not be accessible from nested containers.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docker-build-nix/action.yaml

@@ -87,8 +87,8 @@ runs:
         TARGET_IMAGE="docker://${{ inputs.registry }}/${{ inputs.image-name }}:${{ inputs.image-tag }}"
         echo "Pushing OCI image to: $TARGET_IMAGE"
 
-        # Set TMPDIR to a writable location
-        export TMPDIR="${TMPDIR:-$PWD/tmp}"
+        # Set TMPDIR to a writable location (/tmp should always be writable in containers)
+        export TMPDIR="${TMPDIR:-/tmp}"
         mkdir -p "$TMPDIR"
 
         # Build skopeo command with optional credentials