test: stabilize prompt shell cwd test

packages/opencode/test/session/prompt.test.ts

@@ -1055,6 +1055,7 @@ unix("shell captures stdout and stderr in completed tool output", () =>
 )
 
 unix("shell completes a fast command on the preferred shell", () =>
+  withSh(() =>
     provideTmpdirInstance(
       (dir) =>
         Effect.gen(function* () {
@@ -1076,6 +1077,7 @@ unix("shell completes a fast command on the preferred shell", () =>
         }),
       { git: true, config: cfg },
     ),
+  ),
 )
 
 unix("shell lists files from the project directory", () =>