GCS_DIR is not dynamic

Currently, `GCS_DIR`'s parent directory is set to match the `GOOGLE_CLOUD_PROJECT` string, ref https://github.com/GoogleCloudPlatform/pcap-sidecar/blob/main/scripts/init#L48

It would be nice if either:
- I get to choose a subdirectory in a bucket where pcaps are uploaded (while maintaining the original subpath format `env/service-name/region/etc.`)
- We have a more expressive subdirectory name by default.

Start at scripts/init around line 48 and trace how GCS_DIR is derived from GOOGLE_CLOUD_PROJECT and how the upload path is formed. Clarify whether the intended outcome is user-selected bucket subdirectories or a more expressive default, while preserving the env/service-name/region path format; done means the selected behavior is implemented and its path handling is verified.

Written by the indexing model from the issue text.