Fix compilation with serde 1.0.122

This may or may not be a bug in serde, but let's commit a fix here first
in any case.

Closes #9101

src/cargo/util/config/de.rs

@@ -527,7 +527,7 @@ impl<'de, 'config> de::MapAccess<'de> for ValueDeserializer<'config> {
                 seed.deserialize(Tuple2Deserializer(0i32, path.to_string_lossy()))
             }
             Definition::Environment(env) => {
-                seed.deserialize(Tuple2Deserializer(1i32, env.as_ref()))
+                seed.deserialize(Tuple2Deserializer(1i32, env.as_str()))
             }
             Definition::Cli => seed.deserialize(Tuple2Deserializer(2i32, "")),
         }