refactor(tvix): Very minor lint fixes

Change-Id: I4aca504d98c79f931b7faadff86f40d27ed5a2c5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1719
Tested-by: BuildkiteCI
Reviewed-by: kanepyork <rikingcoding@gmail.com>

third_party/nix/src/libexpr/value.hh

@@ -11,7 +11,7 @@
 
 namespace nix {
 
-typedef enum {
+using ValueType = enum {
   tInt = 1,
   tBool,
   tString,
@@ -27,7 +27,7 @@ typedef enum {
   tPrimOpApp,
   _reserved1,  // formerly tExternal
   tFloat
-} ValueType;
+};
 
 class Bindings;
 struct Env;