nix: fix --store completion

Author: hedning

_nix

@@ -57,6 +57,9 @@ for option_description in $common_options_descriptions; do
     done
 done
 
+# The --store option is undocumented for some reason
+common_options+=("--store[Select a nix store]:Store:->STORE-URI")
+
 local -a main_commands
 # Extract the commands with descriptions
 # like ('command:some description' 'run:run some stuff'):