bug#41052: core-updates: LyX: Errors and messages at start up (no textclasses found)

[sirgazil]

It seems that LyX is missing its default classes, which results in lots of 
messages when starting the application. 

In my case, this happens in core-updates using a virtual machine. I can't 
reproduce this problem in my real machine, maybe because I have some leftovers 
from previous installations of LyX, I don't know.


## Steps to reproduce

1. guix system vm os-config.scm (with GNOME and LyX)
2. Run the resulting virtual machine
3. Log in to the GNOME desktop
4. Press Alt + F2, type lyx2.3 and hit Enter to start LyX


## Expected result

LyX starts without errors and displays the "Welcome to LyX" document.


## Unexpected result

A message about a missing a textclass appears:

https://multimedialib.files.wordpress.com/2020/05/guix-lyx-no-textclasses-message-2020-05-03-1.png

Clicking on "Reconfigure" results in an error message about an article class 
not found:

https://multimedialib.files.wordpress.com/2020/05/guix-lyx-document-class-not-found-error-2020-05-03.png

Clicking "Ok" results in another message. This time about Cite engine not 
available:

https://multimedialib.files.wordpress.com/2020/05/guix-lyx-cite-engine-not-available-message-2020-05-03.png

Clicking "Ok" here finally takes you to the "Welcome to LyX document".

https://multimedialib.files.wordpress.com/2020/05/guix-lyx-welcome-document-2020-05-03.png

At this point, however, I don't want to use the application because I would 
expect many things to fail. I didn't check if there is any workaround (e.g. 
installing packages that provide the missing classes and cite engine).


## System information

```
$ lyx2.3 --version
LyX 2.3.3 (2019-06-10)
CMake Build

$ LANG=C guix describe
Generation 78   May 02 2020 18:00:20    (current)
  guix 7b07852
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: core-updates
    commit: 7b07852ddb334c92bcef69666f21c599f1f0fa79
  sirgazil-x ef6ac93
    repository URL: https://gitlab.com/sirgazil/guix-channel-x.git
    branch: master
    commit: ef6ac9331a7847fdec6f0eb199524b1b755ba0df
```


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