[paths] Add default lookup for config folder at /etc/kibana

When `(KBN|KIBANA)_PATH_CONF` was introduced, we removed the fallback lookup to
`/etc/kibana/kibana.yml` in #67433.  This environment variable was added
to `/etc/default/kibana` and made available, scoped to the Kibana service
for deb and rpm packages.  This environment setting, however, is not available
when running binaries directly and caused the regression noted in

This re-adds a fallback lookup to `/etc/kibana/kibana.yml` when
searching for a kibana.yml configuration.

Testing
1) Install a .deb or .rpm package
2) `sudo -u kibana /usr/share/bin/kibana --version`

Closes #101249

packages/kbn-utils/src/path/index.ts

@@ -18,6 +18,7 @@ const CONFIG_PATHS = [
   process.env.KIBANA_PATH_CONF && join(process.env.KIBANA_PATH_CONF, 'kibana.yml'),
   process.env.CONFIG_PATH, // deprecated
   join(REPO_ROOT, 'config/kibana.yml'),
+  '/etc/kibana/kibana.yml',
 ].filter(isString);
 
 const CONFIG_DIRECTORIES = [