ci: Fix automatic publishing of tags

When a tag is pushed it kicks of a job to publish the proto gem. This job was failing because the target directory did not exist.

This fixes the error as seen in https://gitlab.com/gitlab-org/gitaly/-/jobs/4310397320 :

$ make publish-proto-gem
Warning: the running version of Bundler (2.2.33) is older than the version that created the lockfile (2.3.24). We suggest you to upgrade to the version that created the lockfile by running `gem install bundler:2.3.24`.
Fetching gem metadata from https://rubygems.org/.
Using bundler 2.2.33
Fetching grpc-tools 1.42.0
Installing grpc-tools 1.42.0
Bundle complete! 1 Gemfile dependency, 2 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.
-> git describe --tag
Testing for changed files
-> git diff --quiet --exit-code
Testing for staged changes
-> git diff --quiet --cached --exit-code
-> bundle exec grpc_tools_ruby_protoc -I /builds/gitlab-org/gitaly/proto --ruby_out=/tmp/d20230518-35-zdiat4/ruby/proto/gitaly --grpc_out=/tmp/d20230518-35-zdiat4/ruby/proto/gitaly /builds/gitlab-org/gitaly/proto/blob.proto /builds/gitlab-org/gitaly/proto/cleanup.proto /builds/gitlab-org/gitaly/proto/commit.proto /builds/gitlab-org/gitaly/proto/conflicts.proto /builds/gitlab-org/gitaly/proto/diff.proto /builds/gitlab-org/gitaly/proto/errors.proto /builds/gitlab-org/gitaly/proto/hook.proto /builds/gitlab-org/gitaly/proto/internal.proto /builds/gitlab-org/gitaly/proto/lint.proto /builds/gitlab-org/gitaly/proto/log.proto /builds/gitlab-org/gitaly/proto/namespace.proto /builds/gitlab-org/gitaly/proto/objectpool.proto /builds/gitlab-org/gitaly/proto/operations.proto /builds/gitlab-org/gitaly/proto/praefect.proto /builds/gitlab-org/gitaly/proto/ref.proto /builds/gitlab-org/gitaly/proto/remote.proto /builds/gitlab-org/gitaly/proto/repository.proto /builds/gitlab-org/gitaly/proto/server.proto /builds/gitlab-org/gitaly/proto/service_config.proto /builds/gitlab-org/gitaly/proto/shared.proto /builds/gitlab-org/gitaly/proto/smarthttp.proto /builds/gitlab-org/gitaly/proto/ssh.proto /builds/gitlab-org/gitaly/proto/transaction.proto
-> gem build /tmp/d20230518-35-zdiat4/gitaly.gemspec --output /builds/gitlab-org/gitaly/_build/gitaly.gem
ERROR:  While executing gem ... (Errno::ENOENT)
    No such file or directory @ rb_sysopen - /builds/gitlab-org/gitaly/_build/gitaly.gem
command failed: gem build /tmp/d20230518-35-zdiat4/gitaly.gemspec --output /builds/gitlab-org/gitaly/_build/gitaly.gem
make: *** [Makefile:496: build-proto-gem] Error 1