Disable new signal-based asynchronous goroutine preemption from GO 1.…

…14 in git env (go-gitea#11237)

As seen in trouble shooting go-gitea#11032 the new feature of Go 1.14 is causing several second delays in startup in certain situations. Debugging shows it spending several seconds handling SIGURG commands during init:

```
6922:04:51.984234 trace init() ./modules/queue/unique_queue_wrapped.go
remote: ) = 69 <0.000012>
remote: [pid 15984] 22:04:51 write(1, "\ttime taken: 236.761\302\265s\n\n", 25    time taken: 236.761µs
remote:
remote: ) = 25 <0.000011>
remote: [pid 15984] 22:04:51 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=15984, si_uid=0} ---
remote: [pid 15984] 22:04:52 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=15984, si_uid=0} ---
remote: [pid 15984] 22:04:52 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=15984, si_uid=0} ---
```

This causes up to 20 seconds added to a push in some cases as it happens for each call of the gitea hook command. This is likely the cause of go-gitea#10661 as well and would start to effect users once we release 1.12 which would be the first release compiled with Go 1.14. I suspect this is just a slight issue with the upstream implementatation as there have been a few very similar bugs fixed and reported:

 golang/go#37741
 golang/go#37942

We should revisit this in the future and see if a newer version of Go has solved it, but for now disable this option in the environment that gitea hook runs in to avoid it.

modules/git/command.go

@@ -119,6 +119,8 @@ func (c *Command) RunInDirTimeoutEnvFullPipelineFunc(env []string, timeout time.
 		cmd.Env = env
 		cmd.Env = append(cmd.Env, fmt.Sprintf("LC_ALL=%s", DefaultLocale))
 	}
+
+	cmd.Env = append(cmd.Env, "GODEBUG=asyncpreemptoff=1")
 	cmd.Dir = dir
 	cmd.Stdout = stdout
 	cmd.Stderr = stderr