postinstall - no trace during installation/make develop

[matthieumarrast]

In manager.py, method _postinstall_plugin, the command _plugins.postinstall is called as shell script.
It results that not standard output shown to the user when installing/make_developing the plugin.

It's very annoying to not be aware of postinstall execution. Especially when important commands are run into it... or when the postinstall script takes a long time.

Note: a # FIXME: should be python methods and not shell is present in the file

Example with adding datetime:

[mfserv:~/my_plugin]↥ develop ± make develop
- Checking plugin...                                       [ OK ]              
- Devlinking plugin my_plugin...               [ RUNNING ]   
start;16/01/2023 17:49:13
                                                           [ OK ] 
end:16/01/2023 17:51:14