misc/cgo: invoke "go" from $GOROOT/bin instead of $PATH

misc/cgo: invoke "go" from $GOROOT/bin instead of $PATH

If PATH doesn't contain GOROOT/bin as the first element, this could
otherwise end up running entirely the wrong command (and from the
wrong GOROOT, even).

I pre-tested this change on release-branch.go1.17 using a gomote.
I believe that it will fix the test failure on that branch,
but will need to be backported.

For #52995.

Change-Id: Ib0c43289a1e0ccf9409f0f0ef8046501a955ce65
Reviewed-on: https://go-review.googlesource.com/c/go/+/407294
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>