fix: include CGO_TOOLCHAINS to _go_proto_aspect (#4625)

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**What type of PR is this?**

Bug fix

**What does this PR do? Why is it needed?**

Adds CGO_TOOLCHAINS to the proto aspect so it can propagate down to
go_proto_library targets.

When enabling race detection on a `go_test` that depends on a
`go_proto_library`, the CC toolchains weren't being propagated. This
started breaking when the fallback from
https://github.com/bazel-contrib/rules_go/pull/4599 was removed.

**Which issues(s) does this PR fix?**

Fixes https://github.com/bazel-contrib/rules_go/issues/4624

**Other notes for review**

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Co-authored-by: Fabian Meumertzheim <fabian@meumertzhe.im>