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/__mocks__/input.ts

@@ -10,6 +10,7 @@ export const mockGetFromUser = jest.fn().mockResolvedValue({
   buildMethod: undefined,
   buildVersion: '1.3.37',
   customParameters: '',
+  chownFilesTo: '',
 });
 
 export default {