Make 'nice' check cross-platform

`nice(1)` has no `--version` option in non-GNU installations (FreeBSD, in this case). Rather do a `nice true` to test its function.

Signed-off-by: Eirik Øverby <[email protected]>

lib/Controller/AdminController.php

@@ -201,7 +201,7 @@ public function nice(): JSONResponse {
 		}
 
 		try {
-			exec($nice_path . ' --version' . ' 2>&1', $output, $returnCode);
+			exec($nice_path . ' true' . ' 2>&1', $output, $returnCode);
 		} catch (\Throwable $e) {
 			return new JSONResponse(['nice' => false]);
 		}