cmd/go: ignore irrelevant 'go test' failure in TestGoTestRaceInstallCgo

This test runs 'go test -race -i runtime/race' and checks that it did
not overwrite cmd/cgo.

If GOROOT/pkg is read-only and GOROOT/pkg/$GOOS_$GOARCH_race is not
already populated, as are the conditions if the Go toolchain was
installed from source as root using 'make.bash', then 'go test -race
-i' itself will fail because it cannot install packages to GOROOT/pkg.

However, such a failure is not relevant to the test: even if 'go test
-race -i' fails, we can still verify that it did not incidentally
overwrite cmd/cgo.

Updates #28387
Updates #30316

Change-Id: Iff2f75a0aeb4c926290ac3062c83695604522078
Reviewed-on: https://go-review.googlesource.com/c/go/+/207959


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>