Only install specific yarn when NODE_INSTALLER=yarn

.travis.yml

@@ -16,8 +16,7 @@ branches:
   only:
   - master
   - /^v\d+\.\d+\.\d+/
-before_install:
-- curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 0.23.3
+before_install: if [[ "$NODE_INSTALLER" = "yarn" ]]; then curl -L https://yarnpkg.com/install.sh | bash -s -- --version 0.23.3; fi
 script: ci/script.sh
 after_success: ci/coverage.sh
 notifications:

ci/script.sh

@@ -4,6 +4,5 @@ if [[ "$TRAVIS_OS_NAME" = "linux" ]]; then
     sudo docker build -f ci/Dockerfile . -t electron-forge-ci
     sudo docker run -it electron-forge-ci ci/docker.sh $NODE_INSTALLER
 else
-    if [[ "$NODE_INSTALLER" = "yarn" ]]; then npm i -g yarn; fi
     npm run test-coverage -- --installer=$NODE_INSTALLER
 fi