CI: Fix appending to an array

Parentheses are required to add a new element to an array, not to the
first element of the array.

.github/actions/setup/macos/action.yml

@@ -21,7 +21,7 @@ runs:
         dir_config() {
           local args=() lib var="$1"; shift
           for lib in "$@"; do
-            args+="--with-${lib%@*}-dir=$(brew --prefix $lib)"
+            args+=("--with-${lib%@*}-dir=$(brew --prefix $lib)")
           done
           echo "$var=${args[*]}" >> $GITHUB_ENV
         }