Replace literal with ConfigFilename constant (#4613)

A tiny clean-up where I assume it is a good idea to re-use the common
specification of the `"config.yml"` name.

Author: stefanhaller

pkg/config/app_config.go

@@ -136,7 +136,7 @@ func NewAppConfig(
 }
 
 func ConfigDir() string {
-	_, filePath := findConfigFile("config.yml")
+	_, filePath := findConfigFile(ConfigFilename)
 
 	return filepath.Dir(filePath)
 }