ci: clean untracked files before running postUpgradeTasks

Not sure what changed overnight, but Renovate is now committing .npmrc files that we didn't generate.

Example: https://github.com/angular/angular/pull/61492/files

We should revisit this once we transition away from Yarn.

Author: alan-agius4

renovate.json

@@ -4,6 +4,7 @@
 
   "postUpgradeTasks": {
     "commands": [
+      "git clean -f",
       "yarn install --immutable",
       "yarn update-generated-files"
     ],