ci: avoid subshell invocation on APK version check

Use return code instead of scraping the stdio of `apk version --check`.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

.github/workflows/check-apk-valid-version.yml

@@ -55,7 +55,8 @@ jobs:
                 fi
                 PKG_VERSION=$(grep -E '^PKG_VERSION' "$ROOT/Makefile" | cut -f 2 -d '=')
                 if [ -n "$PKG_VERSION" ]; then
-                  if [[ -n $($GITHUB_WORKSPACE/apk version --check "$PKG_VERSION") ]]; then
+                  $GITHUB_WORKSPACE/apk version --quiet --check "$PKG_VERSION"
+                  if [[ "$?" -gt "0" ]]; then
                     echo "PKG_VERSION is not compatible: $PKG_VERSION"
                     INCOMPATIBLE_VERSION+=" $ROOT"
                   fi