Expose `local_files_dir` config from cloud functions

Currently we cannot deploy terraform configurations using this module from CI. Example: https://gitlab.developers.cam.ac.uk/uis/devops/hr/postbox/postbox-infrastructure/-/jobs/1576639#L97

The root cause is that, when running terraform in CI, we need to use a different local_files_dir which is provided via the TF_VAR_local_files_dir environment variable.

The Cloud Function module exposes a variable for this (https://gitlab.developers.cam.ac.uk/uis/devops/infra/terraform/gcp-function/-/blob/main/variables.tf?ref_type=heads#L130) but there is no equivalent variable in this module which is forwarded to the Cloud Function module.

• Add a local_files_dir variable.
• Pass the value to module.uptime_check_auth_proxy.