rename checks-integration -> checks-impure

I think we should not use this for all integration tests

.gitea/workflows/checks-impure.yaml

@@ -8,4 +8,4 @@ jobs:
     runs-on: nix
     steps:
       - uses: actions/checkout@v3
-      - run: nix run .#checks-integration
+      - run: nix run .#checks-impure

checks/integration/flake-module.nix

@@ -37,7 +37,7 @@
       packages =
         integrationTests // {
           # a script that executes all other checks
-          checks-integration = pkgs.writeShellScriptBin "checks-integration" ''
+          checks-impure = pkgs.writeShellScriptBin "checks-impure" ''
             #!${pkgs.bash}/bin/bash
             set -euo pipefail
             ${lib.concatMapStringsSep "\n" (name: ''