consul: allow non-root Nomad to rewrite token

When a task restarts, the Nomad client may need to rewrite the Consul token, but
it's created with permissions that prevent a non-root agent from writing to
it. While Nomad clients should be run as root (currently), it's harmless to
allow whatever user the Nomad agent is running as to be able to write to it, and
that's one less barrier to rootless Nomad.

Ref: #23859 (comment)

.changelog/24410.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+consul: Fixed a bug where non-root Nomad agents could not recreate a task's Consul token on task restart
+```

client/allocrunner/taskrunner/consul_hook.go

@@ -25,7 +25,7 @@ const (
 
 	// consulTokenFilePerms is the level of file permissions granted on the file in
 	// the secrets directory for the task
-	consulTokenFilePerms = 0440
+	consulTokenFilePerms = 0640
 )
 
 type consulHook struct {