feat(tvix): drop usage of sparseTree in favor of lib.sourceByRegex

We can avoid depending on things outside //tvix by just using a similar
util from nixpkgs.

Change-Id: I9ea3e1f0a8a059ea10caaec173569ba9f316aec6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12247
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/store/build.rs

@@ -25,7 +25,7 @@ fn main() -> Result<()> {
             ],
             // If we are in running `cargo build` manually, using `../..` works fine,
             // but in case we run inside a nix build, we need to instead point PROTO_ROOT
-            // to a sparseTree containing that structure.
+            // to a custom tree containing that structure.
             &[match std::env::var_os("PROTO_ROOT") {
                 Some(proto_root) => proto_root.to_str().unwrap().to_owned(),
                 None => "../..".to_string(),