cmd/go/internal/renameio: include ios in the darwin test-flake mitigation
cmd/go/internal/renameio: include ios in the darwin test-flake mitigation

Because the "ios" build constraint implies "darwin", it is already
included in the general "darwin" flakiness workaround in
cmd/go/internal/robustio. We just need to relax the renameio test
to avoid false-positives there.

I do not expect this change to drive the rate of false-positives down
to zero, but it should at least reduce noise on the build dashboard.

For #42066

Change-Id: Ia33dbd33295fce5b3261b4831f2807ce29b82e65
Reviewed-on: https://go-review.googlesource.com/c/go/+/263777
Trust: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/go,cloudflare/go,golang/go,cloudflare/go,golang/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go