fix: apply SliceFlagSeparator to env var sources

SliceFlagSeparator was ignored when a flag's value came from an
environment variable. The separator config was only applied during
CLI argument parsing (via cmd.set), but PostParse—which handles
env var sources—never called setMultiValueParsingConfig.

Fixes #2262

Author: lawrence3699

command_run.go

@@ -213,6 +213,7 @@ func (cmd *Command) run(ctx context.Context, osArgs []string) (_ context.Context
 	}
 
 	for _, flag := range cmd.allFlags() {
+		cmd.setMultiValueParsingConfig(flag)
 		isSet := flag.IsSet()
 		if err := flag.PostParse(); err != nil {
 			return ctx, err

command_test.go

@@ -4590,6 +4590,24 @@ func TestCommandSliceFlagSeparator(t *testing.T) {
 	r.Equal([]string{"ff", "dd", "gg", "t,u"}, cmd.Value("foo"))
 }
 
+func TestCommandSliceFlagSeparatorFromEnvVar(t *testing.T) {
+	t.Setenv("APP_FOO", "0 1 2")
+
+	cmd := &Command{
+		SliceFlagSeparator: " ",
+		Flags: []Flag{
+			&StringSliceFlag{
+				Name:    "foo",
+				Sources: EnvVars("APP_FOO"),
+			},
+		},
+	}
+
+	r := require.New(t)
+	r.NoError(cmd.Run(buildTestContext(t), []string{"app"}))
+	r.Equal([]string{"0", "1", "2"}, cmd.Value("foo"))
+}
+
 func TestCommandMapKeyValueFlagSeparator(t *testing.T) {
 	cmd := &Command{
 		MapFlagKeyValueSeparator: ":",