HELMFILE_ENVIRONMENT variable (#1425)

Co-authored-by: Pawel Przeszlo <pawel.przeszlo@comarch.pl>

README.md

@@ -420,6 +420,9 @@ GLOBAL OPTIONS:
    --interactive, -i                       Request confirmation before attempting to modify clusters
    --help, -h                              show help
    --version, -v                           print the version
+
+Environment variables:
+  HELMFILE_ENVIRONMENT                     specify the environment name, the command line option '-e' have precedence
 ```
 
 ### sync

main.go

@@ -681,7 +681,10 @@ func (c configImpl) Logger() *zap.SugaredLogger {
 func (c configImpl) Env() string {
 	env := c.c.GlobalString("environment")
 	if env == "" {
-		env = state.DefaultEnv
+		env = os.Getenv("HELMFILE_ENVIRONMENT")
+		if env == "" {
+			env = state.DefaultEnv
+		}
 	}
 	return env
 }