Fix materialization message

[hamishmack]

Strings with context are the ones in the store that cannot be updated with the script (not the other way around).

[hamishmack]

Sanity check:

nix-repl> toString ./.
"/Users/hamish/iohk/haskell.nix"

nix-repl> builtins.hasContext (toString ./.)
false

nix-repl> toString pkgs.nix
"/nix/store/p29gcfammnymrxc372fbn22hhr4jpqk2-nix-2.10.3"

nix-repl> builtins.hasContext (toString pkgs.nix)
true

[michaelpj]

Worth a comment, since apparently it's not obvious!

[hamishmack]

I've changed it to not rely on hasContext. I'm not sure if a directory can have a context without starting with builtins.storeDir (and if it does, does that mean the directory is read only), but if it can the old code was broken anyway (since it assumed the regex would match).