os/exec: TestExtraFiles is flaky on 32-bit linux builders

I've seen several of these:

```
--- FAIL: TestExtraFiles (0.10s)
	exec_test.go:607: Run: exit status 1; stdout "leaked parent file. fd = 66; want 65\n", stderr ""
FAIL
FAIL	os/exec	0.687s
```

on the `linux-386-sid` builder in the past weeks. Here's one:

https://build.golang.org/log/b95e0220b4130af6c1bcfed59ec934568717d0ef