Chown files to a custom user after building (fix file ownership) (#250)

* Chown files to a custom user after building

* Better describe `chownFilesTo` input

As suggested by @webbertakken: https://github.com/game-ci/unity-builder/pull/250#discussion_r624575666

Co-authored-by: Webber Takken <webber@takken.io>

* Simplify chown step in `build.sh`

Co-authored-by: Webber Takken <webber@takken.io>

Co-authored-by: Webber Takken <webber@takken.io>

src/model/input.ts

@@ -85,6 +85,10 @@ class Input {
     return core.getInput('customParameters') || '';
   }
 
+  static get chownFilesTo() {
+    return core.getInput('chownFilesTo') || '';
+  }
+
   static get remoteBuildCluster() {
     return core.getInput('remoteBuildCluster') || '';
   }