[fix] pip install -e: legacy editable install (setup.py develop) is deprecated

From [1]: There is now a standardized mechanism [2] for an installer like pip to
request an editable install of a project.  pip is transitioning to using this
standard only instead of invoking the deprecated `setup.py develop` command.

For backward compatibility, we can use switches:

--use-pep517
  https://pip.pypa.io/en/stable/cli/pip_install/#cmdoption-use-pep517

--no-build-isolation
  https://pip.pypa.io/en/stable/cli/pip_install/#cmdoption-no-build-isolation

- [1] https://github.com/pypa/pip/issues/11457
- [2] https://peps.python.org/pep-0660/

Closes: https://github.com/searxng/searxng/issues/3701
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/build-templates/searxng.rst

@@ -123,7 +123,7 @@ ${fedora_build}
 
        # jump to SearXNG's working tree and install SearXNG into virtualenv
        (${SERVICE_USER})$ cd \"$SEARXNG_SRC\"
-       (${SERVICE_USER})$ pip install -e .
+       (${SERVICE_USER})$ pip install --use-pep517 --no-build-isolation -e .
 
 
 .. END manage.sh update_packages