fix(tvix/castore): use bstr for formatting names in errors

Much friendlier than either Base64 or raw byte slices.

Change-Id: I9b4cdd57c83ddc76c0be8103da4320207657a72b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9622
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/Cargo.nix

@@ -8116,6 +8116,10 @@ rec {
             packageId = "blake3";
             features = [ "rayon" "std" ];
           }
+          {
+            name = "bstr";
+            packageId = "bstr";
+          }
           {
             name = "bytes";
             packageId = "bytes";