Add pre_clone_script setting to helm chart

The current helm chart for gitlab-runner does not allow for adding configuration options to the config.toml.

For example, we have a need to add a "pre_clone_script" such as: config.toml: ... pre_clone_script="umask 0022"

Workaround

Define RUNNER_PRE_CLONE_SCRIPT inside of envVars

envVars:
  - name: RUNNER_PRE_CLONE_SCRIPT
    value: "umask 0022"