build: fix pre-commit installation on the nix-shell

Signed-off-by: Tiago Castro <[email protected]>

nix/shell/ci.nix

@@ -1,14 +1,14 @@
 { pkgs }:
 let
-  inPureNixShell = builtins.getEnv "IN_NIX_SHELL" == "pure";
+  nixShellPurity = builtins.getEnv "IN_NIX_SHELL";
 in
 {
   buildInputs = with pkgs; [
     pre-commit
   ];
 
   shellHook = ''
-    if [ -z "$CI" ] && [ "${toString inPureNixShell}" == "0" ]; then
+    if [ -z "$CI" ] && [ "${nixShellPurity}" == "impure" ]; then
       echo "Installing CI pre-commit hooks..."
       pre-commit install
       pre-commit install --hook commit-msg