cmd/go: don't initialize Builder in envcmd.MkEnv

The Builder isn't needed by MkEnv, and Builder.Init doesn't have side
effects that change the environment. Builder.Init does currently call
CheckGOOSARCHPair, but that's being moved out in CL 234658.

Builder.Init creates the temporary work directory used by the
builder. For the builder created in MkEnv, this directory is never
used. Creating this directory can cause unnecessary errors for
commands that don't use a builder like 'go clean' and 'go list'.

Fixes #38395
Updates #24398

Change-Id: Ib93ae55afdf958000470657f4c4ff5bd92700e46
Reviewed-on: https://go-review.googlesource.com/c/go/+/236563


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