os/exec: older OpenBSD builders failing with EBADF as of CL 171823

[bcmills]

From the openbsd-amd64-62 builder at CL 171823 (https://build.golang.org/log/a179b50c58654a31d09724ba2043d138e8ddbcc6):

testing: can't write /tmp/workdir/tmp/go-build929830143/b061/testlog.txt: write /tmp/workdir/tmp/go-build929830143/b061/testlog.txt: bad file descriptor
FAIL	os/exec	0.941s

CC @ianlancetaylor @mknyszek

[gopherbot]

Change https://golang.org/cl/202445 mentions this issue: os/exec: skip possible netpoll pipe in known FDs in test