Reduced layer count on Windows helper images

What does this MR do?

Reduces layer count for Windows Docker helper images.

Why was this MR needed?

Extra layers in Docker images affect performance. It is critical for Windows where Docker suffers a lot from extra IO operations. Reducing of layer count allows to increase performance without introducing any negative side effects.

Current Dockerfile has tiny commands which yield separate layers:

RUN setx PATH "%PATH%;C:\Program Files\git\cmd;C:\Program Files\git-lfs;C:\Program Files\gitlab-runner-helper" /M
RUN git config --system core.longpaths true
RUN git lfs install --skip-repo

It seems to be overkill and it can be avoided.

Does this MR meet the acceptance criteria?

There is helper image for Windows version 1809 which is built using the proposed Dockerfile:

• https://gitlab.com/kutelev/gitlab-runner/-/jobs/409667898
• registry.gitlab.com/kutelev/gitlab-runner:x86_64-2c703733-servercore1809

This helper image was tried in real build environments and found to work fine.