nixos/tests/acme: Hard code test certificates

The added README.md explains why this has been done.

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+# Fake Certificate Authority for ACME testing
+
+This will set up a test node running [pebble](https://github.com/letsencrypt/pebble)
+to serve ACME certificate requests.
+
+## "Snake oil" certs
+
+The snake oil certs are hard coded into the repo for reasons explained [here](https://github.com/NixOS/nixpkgs/pull/91121#discussion_r505410235).
+The root of the issue is that Nix will hash the derivation based on the arguments
+to mkDerivation, not the output. [Minica](https://github.com/jsha/minica) will
+always generate a random certificate even if the arguments are unchanged. As a
+result, it's possible to end up in a situation where the cached and local
+generated certs mismatch and cause issues with testing.
+
+To generate new certificates, run the following commands:
+
+```bash
+nix-build generate-certs.nix
+cp result/* .
+rm result
+```