Refactor environment variables to configuration and registration (#90)

Close #21.

Refactor environment variables to configuration file (config.yaml) and registration file (.runner).

The old environment variables are still supported, but warning logs will be printed.

Like:

```text
$ GITEA_DEBUG=true ./act_runner -c config.yaml daemon
INFO[0000] Starting runner daemon
WARN[0000] env GITEA_DEBUG has been ignored because config file is used

$ GITEA_DEBUG=true ./act_runner daemon
INFO[0000] Starting runner daemon
WARN[0000] env GITEA_DEBUG will be deprecated, please use config file instead
```

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/90
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>

config/deprecated.go

@@ -0,0 +1,62 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package config
+
+import (
+	"os"
+	"strconv"
+	"strings"
+
+	log "github.com/sirupsen/logrus"
+)
+
+// Deprecated: could be removed in the future. TODO: remove it when Gitea 1.20.0 is released.
+// Be compatible with old envs.
+func compatibleWithOldEnvs(fileUsed bool, cfg *Config) {
+	handleEnv := func(key string) (string, bool) {
+		if v, ok := os.LookupEnv(key); ok {
+			if fileUsed {
+				log.Warnf("env %s has been ignored because config file is used", key)
+				return "", false
+			}
+			log.Warnf("env %s will be deprecated, please use config file instead", key)
+			return v, true
+		}
+		return "", false
+	}
+
+	if v, ok := handleEnv("GITEA_DEBUG"); ok {
+		if b, _ := strconv.ParseBool(v); b {
+			cfg.Log.Level = "debug"
+		}
+	}
+	if v, ok := handleEnv("GITEA_TRACE"); ok {
+		if b, _ := strconv.ParseBool(v); b {
+			cfg.Log.Level = "trace"
+		}
+	}
+	if v, ok := handleEnv("GITEA_RUNNER_CAPACITY"); ok {
+		if i, _ := strconv.Atoi(v); i > 0 {
+			cfg.Runner.Capacity = i
+		}
+	}
+	if v, ok := handleEnv("GITEA_RUNNER_FILE"); ok {
+		cfg.Runner.File = v
+	}
+	if v, ok := handleEnv("GITEA_RUNNER_ENVIRON"); ok {
+		splits := strings.Split(v, ",")
+		if cfg.Runner.Envs == nil {
+			cfg.Runner.Envs = map[string]string{}
+		}
+		for _, split := range splits {
+			kv := strings.SplitN(split, ":", 2)
+			if len(kv) == 2 && kv[0] != "" {
+				cfg.Runner.Envs[kv[0]] = kv[1]
+			}
+		}
+	}
+	if v, ok := handleEnv("GITEA_RUNNER_ENV_FILE"); ok {
+		cfg.Runner.EnvFile = v
+	}
+}