run envsubst as an independent step

.github/workflows/appimage.yml

@@ -59,9 +59,10 @@ jobs:
           --no-man-pages
           --strip-debug
           --compress=1
+      - name: Prepare additional launcher
+        run: envsubst '${SEMVER_STR} ${REVISION_NUM}' < dist/linux/launcher-gtk2.properties > launcher-gtk2.properties
       - name: Run jpackage
         run: >
-          envsubst '${SEMVER_STR} ${REVISION_NUM}' < dist/linux/launcher-gtk2.properties > launcher-gtk2.properties
           ${JAVA_HOME}/bin/jpackage
           --verbose
           --type app-image