fix(tvix/eval/versions): preserve the Number string exactly

This is relevant for builtins.splitVersion:

    nix-repl> builtins.splitVersion "unstable-2022-02-21"
    [ "unstable" "2022" "02" "21" ]

Change-Id: I0a0add178d95d5a82e112b41ed5f3ca5a19608f8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6710
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -220,8 +220,7 @@ fn pure_builtins() -> Vec<Builtin> {
             let parts = s
                 .map(|s| {
                     Value::String(match s {
-                        // TODO(sterni): we should avoid converting back and forth here
-                        VersionPart::Number(n) => format!("{n}").into(),
+                        VersionPart::Number(n) => n.into(),
                         VersionPart::Word(w) => w.into(),
                     })
                 })