Exorcise MORE Darwin test demons because functional1 is /awful/

Yeah uh, I don't think `$shell` is ever set in the functional1
environment. Pretty sure this has been expanding to `''` THIS WHOLE
TIME.

Also `SHELL` is `/sbin/nologin` in that builder. idk why so I'm just
passing `shell` unconditionally in functional's `mkDerivation` helper.

Maybe next we should put `set -euo pipefail` into all these test
builders?

Change-Id: Ie5fb85305932c03a901ba84ffcce48f16a6a6964

tests/functional/config.nix.in

@@ -24,7 +24,7 @@ rec {
 
   mkDerivation = args:
     derivation ({
-      inherit system;
+      inherit system shell;
       builder = shell;
       args = ["-e" args.builder or (builtins.toFile "builder-${args.name}.sh" ''
         if [ -e "$NIX_ATTRS_SH_FILE" ]; then source $NIX_ATTRS_SH_FILE; fi;