[CBM] Cloud backup fails if staging directory is a relative path

What is the problem?
If you pass --obj-staging-dir as a relative path restores fails due to the key being empty. Example from GCP:

Error restoring cluster: failed to execute cluster operations: failed to execute bucket operation for bucket 'beer-sample': failed to transfer bucket data for bucket 'beer-sample': failed to transfer key value data: failed to transfer key value data: failed to stop worker pool: fai

led to open source vBucket 1021: failed to open vBucket 1021: failed to open data store: failed to get object: storage: object name is empty

Backups also fail in a similar way.

The problem originates in PathToKey where we try and get a relative path using the staging directory path and the path parameter. The staging directory path here is a full path to the staging directory, whereas the path parameter is a relative path, meaning filepath.Rel fails, and we ignore the error.

Is there a workaround
Pass an absolute path to --obj-staging-dir

What is the solution?
When the program starts we should turn --obj-staging-dir into an absolute path so that whenever it is used it is an absolute path.