ci: add zizmor check and configuration

`zizmor` is a tool that uses static analysis to find potential security
issues in GitHub Actions [0]. (Yes, it's a bit absurd that GitHub
made a CI system so complicated that tools like this were created, but
I digress.)

Given our increase in GHA usage recently, I think this is a good step
towards keeping our security posture in tip-top shape. (It also keeps
with the theme of automating as many things as possible!)

The rule related to the usages of dangerous-triggers have been disabled
to avoid false-positives. Explanations about the usage of
`pull_request_target` and expectations around its usage can be found in
`.github/workflows/README.md`.

[0]: https://woodruffw.github.io/zizmor/

Co-authored-by: Thomas Gerbet <thomas@gerbet.me>

.github/zizmor.yml

@@ -0,0 +1,12 @@
+# This file defines the ignore rules for zizmor.
+#
+# For rules that contain a high number of false positives, prefer listing them here
+# instead of adding ignore comments. Note that zizmor cannot ignore by line-within-a-string, so
+# there are some ignore items that encompass multiple problems within one `run` block. An issue
+# tracking this is at https://github.com/woodruffw/zizmor/issues/648.
+#
+# For more info, see the documentation: https://woodruffw.github.io/zizmor/usage/#ignoring-results
+
+rules:
+  dangerous-triggers:
+    disable: true

ci/default.nix

@@ -136,6 +136,8 @@ let
             [ "--config=${config}" ];
           includes = [ "*.md" ];
         };
+
+        programs.zizmor.enable = true;
       };
       fs = pkgs.lib.fileset;
       nixFilesSrc = fs.toSource {