[fix] Installation Script install fails (msgspec) (#5353)

Since #5280 has been merged, msgspec, like yaml, is a fixed part of the SearXNG
*settings framework* and therefore, like yaml, must be installed in the virtual
environment before installing SearXNG (``searx``).

The actual reason is that in SearXNG we store settings in the configuration that
are required for the installation of the ``searx`` package.  This means that
these settings (from settings.yml) are read in during the installation, and all the
necessary tools for this (pyyaml, msgspec, setuptools, etc.) must be installed
beforehand (chicken or the egg dilemma).

Related:

- https://github.com/searxng/searxng/pull/5346
- https://github.com/searxng/searxng/pull/5280
- https://github.com/searxng/searxng/pull/5254

Closes: https://github.com/searxng/searxng/issues/5352

docs/build-templates/searxng.rst

@@ -120,6 +120,7 @@ ${fedora_build}
        pip install -U setuptools
        pip install -U wheel
        pip install -U pyyaml
+       pip install -U msgspec
 
        # jump to SearXNG's working tree and install SearXNG into virtualenv
        (${SERVICE_USER})$ cd \"$SEARXNG_SRC\"