steamcompmgr: Enable Gamescope WSI layer by default in nested

Hopefully this has had enough testing and can be used now.

At the very least, this will help us smoke out some issues.

src/steamcompmgr.cpp

@@ -6402,6 +6402,8 @@ spawn_client( char **argv )
 
 		unsetenv( "ENABLE_VKBASALT" );
 
+		// Enable Gamescope WSI by default for nested.
+		setenv( "ENABLE_GAMESCOPE_WSI", "1", 0 );
 		execvp( argv[ 0 ], argv );
 
 		xwm_log.errorf_errno( "execvp failed" );