mediawiki: Fix app installation process doesn't display status information

During 'setup' and 'update' post installation calls, the mediawiki app page doesn't show any status information. Use the call name 'post' instead so that the 'Performing post-install operation' message is displayed on the app page.

Fixes #2024 (closed)

Tested that all the mediawiki app tests pass on first app installation.

Signed-off-by: Veiko Aasa veiko17@disroot.org