cmd/go: simplify code that still assumed the build cache could be nil

cmd/go: simplify code that still assumed the build cache could be nil

cache.Default always returns a non-nil value since Go 1.12; the docs were
updated in https://go.dev/cl/465555.

This updates all the callers of cache.Default that were checking whether
the result was nil so the code isn't misleading/confusing to readers.

Change-Id: Ia63567dd70affef6041c744259f65cea79a2752e
Reviewed-on: https://go-review.googlesource.com/c/go/+/489355
Auto-Submit: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>