Make qgis_enable_plugins("processing") work

[florisvdh]

A side effect of --skip-loading-plugins in some of the GHA workflows (see PR #209) seems to be that the processing provider can be reported as 'disabled' when the list of plugins is queried (qgis/QGIS#57409).

While this does not affect functionality (algorithms of processing are always accessible), its disabled state is reported in a message and more importantly, qgis_enable_plugins() ignores the processing plugin (an intended feature of qgis_disable_plugins()).

While considering the message OK and considering the artifact of a disabled processing plugin a task for QGIS, we should at least make qgis_enable_plugins("processing") work before merging PR #209.

> qgis_configure()
getOption('qgisprocess.path') was not found.
Sys.getenv('R_QGISPROCESS_PATH') was not found.
Trying 'qgis_process' on PATH...
Success!
Now using 'qgis_process' in the system PATH.
>>> If you need another installed QGIS instance, run `qgis_configure()`;
    see `?qgis_configure` if you need to preset the path of 'qgis_process'.

QGIS version is now set to: 3.36.2-Maidenhead
Using JSON for output serialization.
Using JSON for input serialization.
8 out of 12 available processing provider plugins are enabled.
You now have access to 1987 algorithms from 15 QGIS processing providers.

>>> Run `qgis_enable_plugins()` to enable 4 disabled plugins and access their algorithms:
    ViewshedAnalysis, networks, processing, valhalla

Saving configuration to '/home/floris/.cache/R-qgisprocess/cache-0.3.0.9001.rds'
Use qgis_algorithms(), qgis_providers(), qgis_plugins(), qgis_path() and
qgis_version() to inspect the cache environment.

> qgis_enable_plugins("processing")
Ignoring the 'processing' plugin, because it is always available to 'qgis_process' (not QGIS though).
No QGIS plugins to be handled; exiting.