cmd/internal/moddeps: use a temporary directory for GOMODCACHE if needed

CL 328770 should be sufficient to fix the specific failure in the
report, but when attempting to reproduce it I noticed a related
failure mode, triggered by the environment variables set in
src/run.bash.

The failure mode is currently masked on the Go project builders due to
the lack of any 'longtest' builder running as a non-root user
(#10719).

It is also masked from Go contributors running 'run.bash' locally
because 'run.bash' does not actually run all of the tests unless
GO_TEST_SHORT=0 is set in the environment (#29266, #46054).

Fixes #46695

Change-Id: I272c09dae462734590dce59b3d3c5b6d3f733c92
Reviewed-on: https://go-review.googlesource.com/c/go/+/328771


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>