Update Mattermost to fix ENV bug

Package Merge Request

Package Changes

Update the conditional around envs so that siteurl is properly set always.

Package MR

https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost/-/merge_requests/82

For Issue

Closes https://repo1.dso.mil/platform-one/big-bang/bigbang/-/issues/1077