flake: TestReadFile/RelativePath

```
=== FAIL: agent TestReadFile/RelativePath (25.00s)
    coderdtest.go:1592: 
        	Error Trace:	/home/runner/work/coder/coder/coderd/coderdtest/coderdtest.go:1592
        	            				/home/runner/work/coder/coder/agent/files_test.go:203
        	Error:      	Should be true
        	Test:       	TestReadFile/RelativePath
```

seen here: https://github.com/coder/coder/runs/50035973294