Alerting: Fix HCL export for alerts with non-zero "for" field. (#76739)

* Alerting: Fix HCL export for alerts with non-zero "for" field.

Fixes #76734

* fix tests

---------

Co-authored-by: Yuri Tseretyan <yuriy.tseretyan@grafana.com>

pkg/services/ngalert/api/api_provisioning_test.go

@@ -605,7 +605,7 @@ func TestProvisioningApi(t *testing.T) {
 
     no_data_state  = "Alerting"
     exec_err_state = "Error"
-    for            = 0
+    for            = "0s"
     annotations = {
       test = "annotation"
     }
@@ -632,7 +632,7 @@ func TestProvisioningApi(t *testing.T) {
 
     no_data_state  = "OK"
     exec_err_state = "OK"
-    for            = 0
+    for            = "0s"
     is_paused      = false
   }
 }