cmd/go/internal/list: ensure that cfg.BuildMod is initialized before reading it in 'go list -m'
The default value of cfg.BuildMod depends on the 'go' version in the
go.mod file. The go.mod file is read and parsed, and its settings are
applied, in modload.InitMod.
As it turns out, modload.Enabled does not invoke InitMod, so
cfg.BuildMod is not necessarily set even if modload.Enabled returns
true.
Updates #33848
Change-Id: I13a4dd80730528e6f1a5acc492fcfe07cb59d94e
Reviewed-on: https://go-review.googlesource.com/c/go/+/202917
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Jay Conrod <jayconrod@google.com>