fix(licensing): add unityLicensingToolset input + fix HOME/USER under runAsHostUser

Addresses #739. Two opt-in, backwards-compatible changes for users on Linux
Docker builds with a Unity floating-license server:

1. Floating license servers that host multiple toolsets had no way to tell
   the Licensing Client which toolset to request — Unity could fall through
   to an entitlement that lacks build-target support (e.g. Android), then
   silently produce a Linux Standalone artifact. The action now accepts an
   optional unityLicensingToolset input that is written into
   services-config.json. When unset, the rendered config is byte-for-byte
   identical to before.

2. With runAsHostUser: true, su was invoked without explicit HOME/USER, so
   the host user inherited root's environment (HOME=/root, USER unset). The
   Unity Licensing Client, which writes to ~/.config/unity3d, could not
   resolve a writable home directory, leading to intermittent license
   activation failures. Set HOME/USER/LOGNAME explicitly before sourcing
   build steps. Change lives entirely inside the existing runAsHostUser
   branch.

src/model/input.ts

@@ -122,6 +122,10 @@ class Input {
     return Input.getInput('unityLicensingServer') ?? '';
   }
 
+  static get unityLicensingToolset(): string {
+    return Input.getInput('unityLicensingToolset') ?? '';
+  }
+
   static get buildMethod(): string {
     return Input.getInput('buildMethod') ?? ''; // Processed in docker file
   }