This jumps Mattermost ESR Versions (see [1] for their release cycle). The
new version makes use of Go's workspace feature, which unfortunately the
buildGoModule function does not (yet?) support [2], and unfortunately this
breaks the previous build process for mattermost.
Further, the new release also makes use of private modules only included
in the (non-free) enterprise version of mattermost which makes it impossible
to build in the usual way even outside of nixpkgs's build abstractions [3].
Both issues can be solved by using Go 1.22, which has added support for
vendoring when using workspaces, and instructing it to ignore errors with
the -e flag. This requires overriding the go-modules derivation's buildPhase.
Finally, this now also build the commands/mmctl subpackage, which contains
a cli utility to administrate mattermost. This currently has its own nixpkgs
package for no reason i can see at all (it also has a version mismatch
between nixpkgs's mattermost and nixpkgs's mmctl).
[1] https://docs.mattermost.com/upgrade/extended-support-release.html
[2] https://github.com/NixOS/nixpkgs/issues/203039
[3] https://github.com/mattermost/mattermost/issues/26221
