misc/cgo/fortran: avoid writing to $PWD

The bash script that drives this test needs to know whether the
fortran compiler works, but it doesn't actually care about the
generated binary. Write that binary to /dev/null.

Updates #28387
Updates #30316

Change-Id: I4f86da1aeb939fc205f467511fc69235a6a9af26
Reviewed-on: https://go-review.googlesource.com/c/go/+/208124


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>