Set `umask 0000` before executing scripts for k8s executor

What does this MR do?

For kubernetes executor, run umask 0000 before running any scripts so the build folders are created with proper permissions that allow a non-privileged user to write to.

Why was this MR needed?

This is to fix https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/2570 When the runner image is set to use a non-privileged user, kubernetes executor reports permission denied while trying to dump variables such as CI_SERVER_TLS_KEY_FILE to $BUILDS_DIR/${PROJECT}.tmp folder.

Are there points in the code the reviewer needs to double check?

There's already a MR that fixes the issue but I feel changing the permission of files for all executors is a broad change. This MR fixes the issue by only changing what's affected.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Closes #2570 (closed)