fix(mpv).darwin[2]

well, sw_vers didn't take -- until after v12, i see.

projects/mpv.io/package.yml

@@ -81,7 +81,7 @@ test:
   # the swift bits rely on a lib not available on macOS 11; we've seen this before.
   # no point in holding it out just for that, though
   - run: |
-      if test "$(sw_vers --productVersion | cut -d . -f 1)" -lt 12; then
+      if test "$(sw_vers -productVersion | cut -d . -f 1)" -lt 12; then
         exit 0
       fi
     if: darwin