add options to configure aws credentials for private binary cache

[adrian-gierakowski]

This is quite trivial to do on linux since we can just drop aws creds into /root/.aws and dhe daemon picks them up, but on macos as far as I know it requires AWS_SHARED_CREDENTIALS_FILE env var to be set in the nix daemons plist file, which is a PITA.

in fact it would be nice to be able to configure arbitrary env vars to be added to the plist file via this action