feat(schedules): stagger nongnu and gnu-alpha off gnu's slots

README.md

@@ -62,6 +62,11 @@ items:
 - It sends a ping to its healthcheck. The scheduler starts runs, but it does not know the
   result of a run.
 
+The mirror repositories do not contain the times of their runs. This repository is the only
+location of the slots. The cron expression of each healthcheck has the same times as the
+slots of its job, in UTC. When you change the slots of a job, change that cron expression
+right after the host gets the change.
+
 The host gets a change when the host's flake moves its lock to a new commit of this flake.
 A new job starts at the next tick.
 

schedules/gnu-alpha.go

@@ -2,4 +2,4 @@ package schedules
 
 // katoptra/gnu-alpha is a mirror of GNU's alpha release tree. The scheduler starts it twice
 // a day.
-var _ = register(Job{Repo: "katoptra/gnu-alpha", File: "sync.yml", Slots: S3 | S15})
+var _ = register(Job{Repo: "katoptra/gnu-alpha", File: "sync.yml", Slots: S11 | S23})

schedules/nongnu.go

@@ -2,4 +2,4 @@ package schedules
 
 // katoptra/nongnu is a mirror of Savannah's nongnu release tree. The scheduler starts it
 // twice a day.
-var _ = register(Job{Repo: "katoptra/nongnu", File: "sync.yml", Slots: S3 | S15})
+var _ = register(Job{Repo: "katoptra/nongnu", File: "sync.yml", Slots: S7 | S19})