"Cubicweb-ctl upgrade --fs-only" cmd must not performe BD migrations and write new cubes versions in the DB

In case of cubes upgrade cubicweb-ctl upgrade instance --fs-only do write new cubes versions in the base and possibly performes the migrations.

See the traceback bellow on a read-only postgres base:

usr/lib/python3/dist-packages/jinja2/runtime.py:318: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
      from collections import Mapping
    2020-06-05 13:47:02 - (cubicweb.sources.system) CRITICAL: no text index table
    Traceback (most recent call last):
      File "/usr/lib/python3/dist-packages/cubicweb/cwctl.py", line 151, in run_arg
        status = cmdmeth(appid) or 0
      File "/usr/lib/python3/dist-packages/cubicweb/cwctl.py", line 771, in upgrade_instance
        mih.migrate(vcconf, reversed(toupgrade), self.config)
      File "/usr/lib/python3/dist-packages/cubicweb/server/migractions.py", line 172, in migrate
        super(ServerMigrationHelper, self).migrate(vcconf, toupgrade, options)
      File "/usr/lib/python3/dist-packages/cubicweb/migration.py", line 186, in migrate
        self.cube_upgraded(cube, toversion)
      File "/usr/lib/python3/dist-packages/cubicweb/server/migractions.py", line 157, in cube_upgraded
        text_type(version))
      File "/usr/lib/python3/dist-packages/cubicweb/server/migractions.py", line 1352, in cmd_set_property
        prop.cw_set(value=value)
      File "/usr/lib/python3/dist-packages/cubicweb/entity.py", line 1327, in cw_set
        self._cw.execute(rql, qargs)
      File "/usr/lib/python3/dist-packages/cubicweb/server/session.py", line 174, in check_open
        return func(cnx, *args, **kwargs)
      File "/usr/lib/python3/dist-packages/cubicweb/server/session.py", line 719, in execute
        rset = self._execute(self, rql, kwargs, build_descr)
      File "/usr/lib/python3/dist-packages/cubicweb/statsd_logger.py", line 121, in __call__
        return self.callable(*args, **kw)
      File "/usr/lib/python3/dist-packages/cubicweb/server/querier.py", line 565, in execute
        results = plan.execute()
      File "/usr/lib/python3/dist-packages/cubicweb/server/querier.py", line 187, in execute
        result = step.execute()
      File "/usr/lib/python3/dist-packages/cubicweb/server/ssplanner.py", line 525, in execute
        repo.glob_update_entity(cnx, edited)
      File "/usr/lib/python3/dist-packages/cubicweb/server/repository.py", line 846, in glob_update_entity
        source.update_entity(cnx, entity)
      File "/usr/lib/python3/dist-packages/cubicweb/server/sources/native.py", line 597, in update_entity
        self.doexec(cnx, sql, attrs)
      File "/usr/lib/python3/dist-packages/cubicweb/statsd_logger.py", line 121, in __call__
        return self.callable(*args, **kw)
      File "/usr/lib/python3/dist-packages/cubicweb/server/sources/native.py", line 684, in doexec
        cursor.execute(str(query), args)
    psycopg2.errors.ReadOnlySqlTransaction: ERREUR:  ne peut pas exécuter UPDATE dans une transaction en lecture seule
    
    instance francearchivesB not upgraded: ERREUR:  ne peut pas exécuter UPDATE dans une transaction en lecture seule
    
    
    Upgrading the instance francearchivesB
    --------------------------------------
    -> migration needed from 2.6.5 to 2.7.4 for francearchives