misc/cgo/testshared: explicitly set GOBIN (instead of unsetting it)

If GOBIN is set in the GOENV file, then merely unsetting it in the
process environment is not sufficient. We can instead either set GOBIN
explicitly, or disable GOENV explicitly. For now, we (semi-arbitrary)
choose the former.

Fixes #37390

Change-Id: Iec54532c804b70546d695105cd89e9169eac5dbb
Reviewed-on: https://go-review.googlesource.com/c/go/+/220652


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>