cmd/dist: remove extraneous '-i' from 'go test -race' command

At one point (before GOCACHE), the '-i' flag meant, effectively,
“save the intermediate results of this command to make
future commands faster”.

However, now that we require GOCACHE to be enabled everywhere, '-i' no
longer has that meaning: the intermediate results are already saved in
GOCACHE, so the -i flag merely adds extra work (copying or linking
things from GOCACHE into pkg), and also adds additional failure modes
resulting from that extra work (particularly when 'pkg' is read-only).

Since the flag now causes more harm than good, omit it.

Updates #30316

Change-Id: I295b6c0fc460dfc11ffa2a964cbb2a40f2935edc
Reviewed-on: https://go-review.googlesource.com/c/go/+/207962


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>