gha-ci: bump Go version, nits
gha-ci: bump Go version, nits

1. Switch from Go 1.16 to Go 1.x (which should now resolve to Go 1.18.1).

2. Add a comment explaining that the above version is not the actual Go
   version used for testing.

3. Drop -mod=vendor as this is the unconditional default since Go 1.16
   (unless GO111MODULES overrides this).

4. While at it, fix the options order in go test command, putting options
   before the file name.

Tested at https://github.com/kolyshkin/protobuf-go/actions/runs/2342081396

Change-Id: Icc9aa47ddc0d04abb69b48fcb6b021e3e8a98df3
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/406954
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Reviewed-by: Joseph Tsai <ad9e03fbec35106ba742f446a098eef3d5e31d20@digital-static.net>
Reviewed-by: Lasse Folger <c1a9282e9a97d9bcfa578c9e6643dbf90dcebfeb@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go