fix args.paths related argparsing, fixes #6994

argparse: the default action is "store" and that overwrote an already
existing list in args.paths (e.g. from --pattern="R someroot") when it
started to process the positional PATH args.

with "extend" it now extends the existing args.paths with the list of
positional PATH arguments (which can be 0..N elements long, nargs="*").

note: "extend" is new since python 3.8, thus this can only be backported
to 1.2-maint, but not to 1.1-maint.

src/borg/archiver/create_cmd.py

@@ -853,4 +853,4 @@ class CreateMixIn:
         )
 
         subparser.add_argument("name", metavar="NAME", type=NameSpec, help="specify the archive name")
-        subparser.add_argument("paths", metavar="PATH", nargs="*", type=str, help="paths to archive")
+        subparser.add_argument("paths", metavar="PATH", nargs="*", type=str, action="extend", help="paths to archive")