Have pgbackrest wrapper update the environment every so often.

[feikesteenbergen]

Currently, the script relies fully on the environment in which it was started. This removes the possibility of updating a k8s secret, and having the update trickle down to the pgbackrest subcommand.

Possible solution:

• allow a directory including environment variables as an argument to the script
• do an envdir <directory> <command> for every pgbackrest call if the directory was set on the command line