Revert "Back to run-at-load hoping that works"

This reverts commit cccb8bc.

It starts too early, fails, and then aborts. When it fails,
launchd says the service failed to initialize and that the system
is read-only.

src/action/macos/create_nix_hook_service.rs

@@ -182,7 +182,9 @@ impl Action for CreateNixHookService {
 /// This function must be able to operate at both plan and execute time.
 async fn generate_plist(service_label: &str) -> Result<LaunchctlHookPlist, ActionErrorKind> {
     let plist = LaunchctlHookPlist {
-        run_at_load: true,
+        keep_alive: KeepAliveOpts {
+            successful_exit: false,
+        },
         label: service_label.into(),
         program_arguments: vec![
             "/bin/sh".into(),
@@ -201,11 +203,17 @@ async fn generate_plist(service_label: &str) -> Result<LaunchctlHookPlist, Actio
 pub struct LaunchctlHookPlist {
     label: String,
     program_arguments: Vec<String>,
-    run_at_load: bool,
+    keep_alive: KeepAliveOpts,
     standard_error_path: String,
     standard_out_path: String,
 }
 
+#[derive(Deserialize, Clone, Debug, Serialize, PartialEq)]
+#[serde(rename_all = "PascalCase")]
+pub struct KeepAliveOpts {
+    successful_exit: bool,
+}
+
 #[non_exhaustive]
 #[derive(Debug, thiserror::Error)]
 pub enum CreateNixHookServiceError {