cmd/internal/script: new hook for adding in toolchain script conditions
Introduce a new function AddToolChainScriptConditions that augments a
default "script.Cond" set with a collection of useful conditions,
including godebug/goexperiment, cgo, race support, buildmode, asan,
msan, and so on. Having these conditions available makes it easier to
write script tests that deal with specific build-flavor corner cases.
The functions backing the new conditions are helper functions migrated
over from the Go command's script test setup.
Updates #68606.
Change-Id: I14def1115b54dc47529c983abcd2c5ea9326b9de
Reviewed-on: https://go-review.googlesource.com/c/go/+/601715
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com>