CI: Another attempt to fix passing an environment variable to a script

.travis.yml

@@ -22,7 +22,7 @@ jobs:
       script: skip
     - stage: build
       script:
-        - TW5_BUILD_TIDDLYWIKI='./node_modules/tiddlywiki/tiddlywiki.js'
+        - export TW5_BUILD_TIDDLYWIKI='./node_modules/tiddlywiki/tiddlywiki.js'
         - ./bin/build-site.sh
     - stage: deploy
       script: ./bin/deploy-site.sh