cmd/go/internal/modload: don't initialize build cache

modload.Init initialized the build cache with the intent of providing
a better error message in Go 1.12, when the build cache became
mandatory (in module mode, packages aren't installed outside the build
cache). Unfortunately, this didn't provide a more descriptive error
(the cache calls base.Fatalf with its own message), and it caused
errors for commands that don't use the cache (like 'go mod edit').

This CL removes the cache initialization from modload.Init. The
builder will initialize it when it's needed.

For #39882

Change-Id: Ibc01ae4e59358dcd08a07ffc97bf556514d0366f
Reviewed-on: https://go-review.googlesource.com/c/go/+/240548


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>