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>

action.yml

@@ -106,6 +106,10 @@ inputs:
 
       Parameters must start with a hyphen (-) and may be followed by a value (without hyphen).
       Parameters without a value will be considered booleans (with a value of true).
+  chownFilesTo:
+    required: false
+    default: ''
+    description: 'User and optionally group (user or user:group or uid:gid) to give ownership of the resulting build artifacts'
   allowDirtyBuild:
     required: false
     default: ''