chore: split everything up into their own files & add a bunch of tests

lib/jobPatch.nix

@@ -0,0 +1,49 @@
+{
+  lib,
+  pkgs,
+  ...
+}: let
+  cilib = import ./. {inherit lib pkgs;};
+  inherit (lib) toList;
+  inherit (cilib.helpers) prependToBeforeScript appendToAfterScript filterJobVariables;
+in {
+  mkJobPatched = {
+    key,
+    job,
+    pipeline_name,
+  }:
+    builtins.removeAttrs (
+      (prependToBeforeScript [
+          "source setup_nix_ci \"gitlab-ci:pipeline:${pipeline_name}:job-deps:${key}\""
+        ]
+        (appendToAfterScript [
+            "finalize_nix_ci"
+          ]
+          job))
+      // lib.optionalAttrs job.nix.enable (
+        (let
+          variables =
+            (filterJobVariables false job)
+            // lib.optionalAttrs job.nix.enable-runner-cache {
+              NIX_CI_CACHE_STRATEGY = "runner";
+            };
+        in
+          # filter empty variables
+          lib.optionalAttrs (variables != {}) {
+            inherit variables;
+          })
+        // (let
+          cache =
+            (toList (job.cache or []))
+            ++ (lib.optional (job.nix.enable-runner-cache) {
+              key = job.nix.runner-cache-key;
+              paths = [".nix-cache/"];
+            });
+        in
+          # filter empty cache
+          lib.optionalAttrs (cache != []) {
+            inherit cache;
+          })
+      )
+    ) ["nix"];
+}