Fix panic related to env values from files and sprig dict funcs (#625)

* fix: 0.68.0: go panic w/ multiple bases/layers

Fixes #623

* fix: 0.64.1+: getOrNil/hasKey fails on Environment.Values with nested maps

Fixes #624

pkg/maputil/maputil.go

@@ -0,0 +1,50 @@
+package maputil
+
+import "fmt"
+
+func CastKeysToStrings(s interface{}) (map[string]interface{}, error) {
+	new := map[string]interface{}{}
+	switch src := s.(type) {
+	case map[interface{}]interface{}:
+		for k, v := range src {
+			var str_k string
+			switch typed_k := k.(type) {
+			case string:
+				str_k = typed_k
+			default:
+				return nil, fmt.Errorf("unexpected type of key in map: expected string, got %T: value=%v, map=%v", typed_k, typed_k, src)
+			}
+
+			var casted_v interface{}
+			switch typed_v := v.(type) {
+			case map[interface{}]interface{}:
+				tmp, err := CastKeysToStrings(typed_v)
+				if err != nil {
+					return nil, err
+				}
+				casted_v = tmp
+			default:
+				casted_v = typed_v
+			}
+
+			new[str_k] = casted_v
+		}
+	case map[string]interface{}:
+		for k, v := range src {
+			var casted_v interface{}
+			switch typed_v := v.(type) {
+			case map[interface{}]interface{}:
+				tmp, err := CastKeysToStrings(typed_v)
+				if err != nil {
+					return nil, err
+				}
+				casted_v = tmp
+			default:
+				casted_v = typed_v
+			}
+
+			new[k] = casted_v
+		}
+	}
+	return new, nil
+}