Fix Workhorse failing on 64-bit unaligned access on Raspberry Pi 32-bit

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/177057 added new
fields to the Redis KeyWatcher structure that caused a `panic:
unaligned 64-bit atomic operation` on Raspberry Pi 32-bit builds for
Workhorse.

I was able to reproduce this by on a Raspberry Pi 64-bit system:

1. Install the 32-bit libraries:
   `apt install libc6:armhf libstdc++6:armhf`.

2. Build `gitlab-workhorse` with `GOARCH=arm` and `GOARM=7`
   environment variable set.

3. Spin up a Redis server.

4. Run `gitlab-workhorse` with this `config.toml`:

```
[redis]
URL = "redis://localhost:6379"
```

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/514010

Changelog: fixed

workhorse/internal/redis/keywatcher.go

@@ -19,11 +19,12 @@ import (
 
 // KeyWatcher is responsible for watching keys in Redis and notifying subscribers.
 type KeyWatcher struct {
+	// Put this field first to ensure backoff.Backoff is aligned for 64-bit access
+	reconnectBackoff backoff.Backoff
 	mu               sync.Mutex
 	newSubscriber    chan struct{}
 	subscribers      map[string][]chan string
 	shutdown         chan struct{}
-	reconnectBackoff backoff.Backoff
 	redisConn        *redis.Client // can be nil
 	conn             *redis.PubSub
 	firstRun         bool