[keyserver] Clean up scp-backup.sh using ShellCheck

Summary: Related Linear issue here. This is part of a set of diffs that will allow ShellCheck to be added to the CI. See inline comments for specific details of the ShellCheck error/warning output.

Test Plan:
ShellCheck

Additionally, this script is run during the keyserver build so if that workflow continues to pass CI we can reasonably assume that the script continues to work as expected.

Reviewers: atul, jon, tomek, ashoat

Reviewed By: atul, ashoat

Subscribers: adrian

Differential Revision: https://phab.comm.dev/D4713