Build and test only changed packages (#352)

* #285 build and test only changed packages

* changed since flag param

* fix build cause --since lerna flag

* fix build cause --since lerna flag

* removed checkout option

* fix build cause --since lerna flag

* fix build cause --since lerna flag

* fix build in progress

* fix build in progress

* removed --since flag for build executions

* removed --since flag for build executions

* changed action checkout step

* added new ref to the action checkout

* removed ref prop from the action checkout. Added new step to fetch branches to compare when using lerna --since

* changed --since param value

* added --since flag to the build and renamed the fetch step

* removed --since flag from build. Added new config to the checkout step workflow

* removed --since flag from yarn build. Added new config to the checkout step workflow

* removed --since flag from yarn build. Added new step to checkout branch master

* Added new step to checkout branch master

package.json

@@ -7,7 +7,7 @@
   "scripts": {
     "start": "yarn build && yarn workspace @spotify-backstage/app start",
     "build": "lerna run build",
-    "test": "cross-env CI=true lerna run test -- --coverage",
+    "test": "cross-env CI=true lerna run test --since origin/master -- --coverage",
     "create-plugin": "backstage-cli create-plugin",
     "lint": "lerna run lint",
     "storybook": "yarn workspace @spotify-backstage/core storybook",