go/internal/srcimporter: use the 'go' command from the Importer's GOROOT
We have no guarantee in general that there is any 'go' command in
$PATH at all, let alone the correct one. However, we can expect that
if a 'go' command is not in scope, the Importer should have a correct
GOROOT setting: otherwise, it would not be able to import anything
from 'std' at all.
Given that information, when we run `go tool cgo` we should use
GOROOT/bin/go specifically, not whatever 'go' we find in $PATH.
This fixes a failure in go/types.TestStdlib that manifests as a
timeout in when the 'go' command is not present in $PATH, due to
repeated retries for every package that transitively depends on
runtime/cgo.