Disable nix-shell purity in stack.yaml

 To allow stack to pick git for user environnement.
 (waiting for input-output-hk/haskell.nix#121 for a better fix.

Author: jbgi

stack.yaml

@@ -100,4 +100,5 @@ extra-deps:
 
 nix:
   enable: true
+  pure: false
   shell-file: stack-shell.nix