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>

misc/cgo/fortran/test.bash

@@ -28,7 +28,7 @@ case "$FC" in
   ;;
 esac
 
-if ! $FC helloworld/helloworld.f90 -o main.exe >& /dev/null; then
+if ! $FC helloworld/helloworld.f90 -o /dev/null >& /dev/null; then
   echo "skipping Fortran test: could not build helloworld.f90 with $FC"
   exit 0
 fi