Use command to avoid name collision with shell builtins

Co-authored-by: Mislav Marohnić <hi@mislav.net>

Author: uzxmx

bin/ruby-build

@@ -475,7 +475,7 @@ fetch_tarball() {
  elif [ -n "$RUBY_BUILD_MIRROR_PACKAGE_URL" ]; then
  mirror_url="$RUBY_BUILD_MIRROR_PACKAGE_URL"
  elif [ -n "$RUBY_BUILD_MIRROR_CMD" ]; then
- mirror_url="$("$RUBY_BUILD_MIRROR_CMD" "$package_url" "$checksum")"
+ mirror_url="$(command "$RUBY_BUILD_MIRROR_CMD" "$package_url" "$checksum")"
  fi
  fi