Replace netrc files with for authentication during the clone step

drone should move away from using netrc files for authentication during the clone step to better support runners that don’t just use the drone/git docker image for this step.

.netrc is not natively supported in every distribution and requires the git-credential-netrc helper, which is not part of the main git suite.

Instead of the .netrc file, the authorization information should be put into the .git-credentials file, which can be read by the builtin git-credential-store credential helper.

An alternative would be to use the git-credential-cache helper (due to socket use only available under linux), so no credential files need to be stored on disk at all. The runner could just pre-fill the in-memory cache with the “git credential approve” command.

In any case, the git credential helper configuration of the clone repo should probably be made explicit and / or configurable in the process (to support eg. both store and cache helper).