Incorrect permissions logic in snap wrapper

On restarting the Prometheus snap, you'll be treated to a lovely spew of "Operation not permitted" errors. For example:

Oct 13 01:22:18 juju-dd628a-stg-is-prometheus-3 prometheus.prometheus[1049870]: chmod: changing permissions of '/var/snap/prometheus/common/tsdb': Operation not permitted
Oct 13 01:22:18 juju-dd628a-stg-is-prometheus-3 prometheus.prometheus[1049871]: chmod: changing permissions of '/var/snap/prometheus/common/tsdb/01GF6ZYKFH7TJN3D3A86BXKWXF': Operation not permitted
Oct 13 01:22:18 juju-dd628a-stg-is-prometheus-3 prometheus.prometheus[1049872]: chmod: changing permissions of '/var/snap/prometheus/common/tsdb/01GF6ZYKFH7TJN3D3A86BXKWXF/chunks': Operation not permitted
Oct 13 01:22:18 juju-dd628a-stg-is-prometheus-3 prometheus.prometheus[1049873]: chmod: changing permissions of '/var/snap/prometheus/common/tsdb/01GF7DP1ZJJDPTDSBTA0B4WJHH': Operation not permitted
Oct 13 01:22:18 juju-dd628a-stg-is-prometheus-3 prometheus.prometheus[1049874]: chmod: changing permissions of '/var/snap/prometheus/common/tsdb/01GF7DP1ZJJDPTDSBTA0B4WJHH/chunks': Operation not permitted
Oct 13 01:22:18 juju-dd628a-stg-is-prometheus-3 prometheus.prometheus[1049875]: chmod: changing permissions of '/var/snap/prometheus/common/tsdb/01GF3ZTE2XAJJ9254VT028KK5W': Operation not permitted
Oct 13 01:22:18 juju-dd628a-stg-is-prometheus-3 prometheus.prometheus[1049876]: chmod: changing permissions of '/var/snap/prometheus/common/tsdb/01GF3ZTE2XAJJ9254VT028KK5W/chunks': Operation not permitted

There are two issues here:

1) We aren't catering for a scenario where we need to modify permissions once ownership is already altered.

2) We're being overzealous with the permissions being applied. The intention was to ensure that moving to a rootless daemon didn't prevent the snap from reading any of its own files, and for that intention to be met there's no need for us to modify permissions within the TSDB.