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

lib/jobDeps.nix

@@ -0,0 +1,36 @@
+{
+  lib,
+  pkgs,
+  ...
+}: let
+  cilib = import ./. {inherit lib pkgs;};
+  inherit (cilib.helpers) filterJobVariables stdenvMinimal;
+in {
+  mkJobDeps = {
+    key,
+    job,
+  }: let
+    variablesWithStorePaths = filterJobVariables true job;
+    variableExports = lib.concatLines (
+      lib.mapAttrsToList (name: value: "export ${name}=\"${value}\"") variablesWithStorePaths
+    );
+    script = ''
+      export PATH="${lib.makeBinPath (job.nix.deps or [])}:$PATH";
+      # variables containing nix derivations:
+      ${variableExports}
+    '';
+  in
+    stdenvMinimal.mkDerivation {
+      name = "gitlab-ci-job-deps-${key}";
+      dontUnpack = true;
+      installPhase =
+        # sh
+        ''
+          echo '${script}' > $out
+          chmod +x $out
+        '';
+      passthru = {
+        inherit script;
+      };
+    };
+}