refactor: Move CI setup to separate Nix file

Instead of polluting the repository namespace with the list of CI
projects, move that to a separate file.

Currently the list of projects to be built by CI is still hardcoded,
but this will be fixed soon.

ci-builds.nix

@@ -0,0 +1,17 @@
+# This file is invoked by the CI build and recursively filters the
+# package set for attributes that should be built automatically.
+#
+# Packages can be opted-in to being built by CI by setting
+# `meta.enableCI = true`.
+#
+# TODO(tazjin): Actually implement the above.
+
+let
+  pkgs = import ./default.nix;
+in with pkgs; [
+  services.tazblog
+  services.nixcon-demo
+  tools.kms_pass
+  tools.blog_cli
+]
+