The iOS exec wrapper copies the environment variables to the binary
running on the device. However, some variables such as HOME, TMPDIR and
GOCACHE refer to directories that might not be valid on the device.

The wrapper already ignores HOME and TMPDIR, but after GO111MODULE
was flipped to on for the standard library, cmd/go tests started
failing without a valid and writable GOCACHE.

It took a while to reproduce because go test does not set an explicit
GOCACHE but cmd/dist test does.

Fixes #30914

Change-Id: If186cddc5cfd7ad39a0b3eb95f9c64a7d53a27e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/168557
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>