fix: Fixed ah pr linter pr target bug

[TannerGilbert]

Currently the ah_lint_pr action fails at the copy_files step with the following error message:

Run # Copy artifacthub-pkg.yml and parent folder into new folder
  # Copy artifacthub-pkg.yml and parent folder into new folder
  mkdir changed_services && cp -pv --parents folder/1.0.0/artifacthub-pkg.yml changed_services
  shell: /usr/bin/bash -e {0}
cp: failed to get attributes of 'folder': No such file or directory
Error: Process completed with exit code 1.

After doing some debugging it turns out that this only happens for pull_request_target and works fine with pull_request.

When using pull_request_target the new service isn't fetched by actions/checkout@v2 even though the get-changed-files action seems to fetch the data without a problem.

This seems to be a problem with the actions/checkout@v2 when working with pull_request_target. To fix this a ref needed to be added as described in actions/checkout#518 (comment).

[Raffy23]

LGTM

[christian-kreuzberger-dtx]

LGTM