vendor-install: remember VENDOR_DIR is a variable

Library/Homebrew/cmd/vendor-install.sh

@@ -115,7 +115,7 @@ fetch() {
     sha="$(ruby -e "require 'digest/sha2'; digest = Digest::SHA256.new; File.open('$CACHED_LOCATION', 'rb') { |f| digest.update(f.read) }; puts digest.hexdigest")"
   # Pure Perl SHA256 implementation
   else
-    sha="$(VENDOR_DIR/sha256 "$CACHED_LOCATION")"
+    sha="$($VENDOR_DIR/sha256 "$CACHED_LOCATION")"
   fi
 
   if [[ "$sha" != "$VENDOR_SHA" ]]