leader-set fails even if is_leader returned True

Hello,

This is probably a duplicate of LP#1723184 but opening a new one.

We had an issue with a juju 2.8.7 controller and model related to that.

The relevant pastebins are:

* https://pastebin.canonical.com/p/vBpcT6ZkSM/ for the charm side, which shows that the leader-set ended up with a permission denied.
The code can be found at https://bazaar.launchpad.net/~ubuntu-repository-cache-charmers/ubuntu-repository-cache/layer-ubuntu-repository-cache/view/293.1.1/reactive/ubuntu-repository-cache.py#L218

We can see that the code check for the leader with
```
    if not hookenv.is_leader():
        LOG('leader-elected fired. This is not the leader')
        return
```
However, we can see in the logs line 73 that this condition passed, resulting in
`2020-12-27 00:04:25 INFO juju-log leader-elected fired. This unit is the new leader: ubuntu-repository-cache/2`
to fail right after on the leader-set.

* The controller units' logs around that time can be found at https://pastebin.canonical.com/p/YBcdCpgjFQ/

Cheers,