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/aws.ts

@@ -149,6 +149,10 @@ class AWS {
             name: 'CUSTOM_PARAMETERS',
             value: buildParameters.customParameters,
           },
+          {
+            name: 'CHOWN_FILES_TO',
+            value: buildParameters.chownFilesTo,
+          },
           {
             name: 'BUILD_TARGET',
             value: buildParameters.platform,