Fix --webroot-path action (#10509)

Fixes #10506.

When --webroot-path was specified multiple times, Certbot was erroring
with `DNSName SAN compared to non-SAN`. That's because, in the
_WebrootPathAction that builds `namespace.webroot_path`, we were passing
`domain` (type `san.DNSName`) as the keys. The other code that modifies
or accesses `namespace.webroot_path` expects the keys to be of type
`str`. In particular `webroot.Authenticator._set_webroots` does:

```python
            for achall in achalls:
                self.conf("map").setdefault(achall.domain, webroot_path)
```

Where `achall.domain` is a `str`.

Two existing unittests would have caught this: `test_multiwebroot` and
`test_webroot_map_partial_without_perform`. However, they faked out the
parsing of the `--domains` flag, and that faked out code was not updated
in #10468. Since this bug is caused by an interaction between the types
produced by the `--domains` flag and those produced by the
`--webroot-path` flag, the tests failed to catch the problem. I've
updated the tests and confirmed that they fail before the fix is
applied.

newsfragments/10509.fixed

@@ -0,0 +1 @@
+Fixed a regression that caused certbot to crash if multiple --webroot-path values were set on the command line.