The XOAUTH2 support was recently added in 47eda8e00a but apparently it
causes regressions when using other SASL methods.
An example error message:
```
IMAP command 'AUTHENTICATE XOAUTH2 SOMEHASH=' returned an error: NO [AUTHENTICATIONFAILED] Invalid credentials (Failure)
```
The cause seems to be that overriding `SASL_PATH` drops all available
SASL mechanisms from `cyrus_sasl`, so only `XOAUTH2` (and `EXTERNAL`)
will be left. See [0] and the following comments for more details.
We'd need to set `SASL_PATH` to a combination/merge of
`${cyrus_sasl}/lib/sasl2` and `${cyrus-sasl-xoauth2}/lib/sasl2`.
Anyway, it seems best to disable the XOAUTH2 support by default due to
the two other concerns mentioned in the comment.
[0]: https://github.com/NixOS/nixpkgs/pull/235148#issuecomment-1575432154
Reported-by: Dominik Schrempf <dominik.schrempf@gmail.com>
fe02c5ee1a