GCS_DIR is not dynamic

[thomasmburke]

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.