Now that modules are always on, cmd/go tests require a valid
GOCACHE. However, on iOS where the go tool is not available, the
cmd/go test driver ends up setting GOCACHE to the empty string.
Fix it by falling back to the builtin default cache directory.

The iOS exec wrapper passes the environment variables to the app
on the device, including $HOME used for the default cache directory.
Skip $HOME to let the device specific and writable $HOME be used
instead.

Should fix cmd/go on the iOS builders that broke when GO111MODULE
defaulted to on.

Change-Id: I0939f5b8aaa1d2db95e64c99f4130eee2d0b4d4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/167938
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>