cmd/go: fix TestScript/cgo_path, cgo_path_space when CC set

These tests failed if CC was set to a path containing a separator
during make.bash. They now set CC explicitly.

Fixes #43897

Change-Id: Ic6e7f192fcb363f0ac9f45b329113255453bf76f
Reviewed-on: https://go-review.googlesource.com/c/go/+/286292


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>