Support go 1.18

At the moment Gitaly doesn't compile with Go v1.18:

error obtaining VCS status: exit status 63
Use -buildvcs=false to disable VCS stamping.

I have a temporary workaround in my config.mak:

GO_MINOR := $(wordlist 2,2,$(subst ., ,$(strip $(shell go env GOVERSION))))

ifeq (${GO_MINOR},18)
    export GOFLAGS := -buildvcs=false
endif

We should check again when https://go-review.googlesource.com/c/go/+/400454 lands in a release (looking at https://github.com/golang/go/issues/51798, I think it will be 1.18.2).