Gitaly requires go 1.12

Job #366635083 failed for 9fe268c9:

 # go install
 # gitlab.com/gitlab-org/gitaly/internal/metadata/featureflag
 internal/metadata/featureflag/grpc_header.go:38:42: undefined: strings.ReplaceAll
 note: module requires Go 1.12Makefile:51: recipe for target 'build' failed
 make[1]: *** [build] Error 2
 make[1]: Leaving directory '/builds/gitlab-org/gitlab/tmp/tests/gitaly/_build'
 make: *** [build] Error 2
 Makefile:25: recipe for target 'build' failed

It looks like out image does not have go 1.12