isync: Disable the XOAUTH2 support by default as it caused regressions

The XOAUTH2 support was recently added in 47eda8e 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]: NixOS#235148 (comment)

Reported-by: Dominik Schrempf <dominik.schrempf@gmail.com>

pkgs/tools/networking/isync/default.nix

@@ -1,7 +1,9 @@
 { lib, stdenv, fetchurl, pkg-config, perl
 , openssl, db, cyrus_sasl, zlib
 , Security
-, withCyrusSaslXoauth2 ? true, cyrus-sasl-xoauth2, makeWrapper
+# Disabled by default as XOAUTH2 is an "OBSOLETE" SASL mechanism and this relies
+# on a package that isn't really maintained anymore:
+, withCyrusSaslXoauth2 ? false, cyrus-sasl-xoauth2, makeWrapper
 }:
 
 stdenv.mkDerivation rec {