chore(tvix/store): use lazy_static and dummy digests

While there's currently nothing in here checking the size of the digest,
we should use something that passes the to-be-introduced validate()
function.

Change-Id: I0c515d9e3afc79292dedebce659a32485aa3d936
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7649
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/Cargo.nix

@@ -6167,6 +6167,10 @@ rec {
             packageId = "blake3";
             features = [ "rayon" "std" ];
           }
+          {
+            name = "lazy_static";
+            packageId = "lazy_static";
+          }
           {
             name = "prost";
             packageId = "prost";