sandbox: flaky unix socket test

[achille-roussel]

https://github.com/stealthrocket/timecraft/actions/runs/5817106556/job/15771258631

--- FAIL: TestConn (61.40s)
    --- FAIL: TestConn/unix.sock (60.38s)
        --- FAIL: TestConn/unix.sock/ConcurrentMethods (60.05s)
            conntest.go:57: test timed out; terminating pipe
            conntest.go:[39](https://github.com/stealthrocket/timecraft/actions/runs/5817106556/job/15771258631#step:8:40)1: unexpected Read error: read unix @->/tmp/TestConnunix.sock148[40](https://github.com/stealthrocket/timecraft/actions/runs/5817106556/job/15771258631#step:8:41)90768/011/unix.sock: use of closed file
            conntest.go:391: unexpected Write error: write unix @->/tmp/TestConnunix.sock1484090768/011/unix.sock: write : file already closed
            conntest.go:392: roundtrip SetDeadline error: set unix @->/tmp/TestConnunix.sock1484090768/011/unix.sock: use of closed file
            conntest.go:392: roundtrip Write error: write unix @->/tmp/TestConnunix.sock1484090768/011/unix.sock: write : file already closed
            conntest.go:392: roundtrip Read error: read unix @->/tmp/TestConnunix.sock1484090768/011/unix.sock: use of closed file
FAIL
FAIL	github.com/stealthrocket/timecraft/internal/sandbox	61.659s