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.