feat(ops/besadii): Refactored tool to trigger sourcehut builds

Refactors //ops/sync-gcsr which was previously responsible for
synchronising the git repository between GCSR and the git.tazj.in cgit
instance to simply be responsible for triggering builds on sourcehut.

This program is intended to run as a git post-update hook.

Note: Not yet feature complete, as interpolation of concrete git
values and also sourcehut secrets is missing.

ops/besadii/default.nix

@@ -0,0 +1,8 @@
+# This program is used as a git post-update hook to trigger builds on
+# sourcehut.
+{ depot, ... }:
+
+depot.buildGo.program {
+  name = "besadii";
+  srcs = [ ./main.go ];
+}