fix: ensure /var/tmp exists for skopeo operations

Skopeo may use /var/tmp for temporary files during docker-archive
operations, even when TMPDIR is set. Create the directory to prevent
'no such file or directory' errors.

docker-build-nix/action.yaml

@@ -91,6 +91,9 @@ runs:
         export TMPDIR="${TMPDIR:-/tmp}"
         mkdir -p "$TMPDIR"
 
+        # Ensure /var/tmp exists as fallback (skopeo may use this)
+        sudo mkdir -p /var/tmp 2>/dev/null || mkdir -p /var/tmp || true
+
         # Build skopeo command with optional credentials
         SKOPEO_CMD="skopeo copy"
         if [ -n "${{ inputs.registry-username }}" ] && [ -n "${{ inputs.registry-password }}" ]; then