feat(tvix/glue): drive builds on IO

That's one possible abstraction to drive builds.
Whenever IO into a store path is requested, we look up the root node,
and in case we don't have it in PathInfoService, but KnownPaths gives us
a Derivation for that output path, trigger a build and await the result.

This recursively might trigger builds for parent paths if they haven't
been built yet.

Another option would be to simply expose a PathInfoService interface for
a builder too, and loop all building into IO via PathInfoService
composition - but let's start with something.

Note tvix-cli doesn't have a configurable BuildService yet, it's plugged
to the DummyBuildService, so whenever it needs to do a build, it'll fail,
but that's how it can be provoked:

```
(builtins.readFile (import <nixpkgs> {}).hello.outPath + "/bin/hello")
[…]
error[E029]: I/O error: /nix/store/cg8a576pz2yfc1wbhxm1zy4x7lrk8pix-hello-2.12.1: builds are not supported with DummyBuildService
 --> [code]:1:2
  |
1 | (builtins.readFile (import <nixpkgs> {}).hello.outPath + "/bin/hello")
  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

Note how this fails, while pure output path calculation
(`(import <nixpkgs> {}).hello.outPath + "/bin/hello")`) still succeeds.

Change-Id: Id2075d8a2b18554d0dd608b4b29146a8cd411e7f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10793
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/glue/Cargo.toml

@@ -4,9 +4,11 @@ version = "0.1.0"
 edition = "2021"
 
 [dependencies]
+async-recursion = "1.0.5"
 bstr = "1.6.0"
 bytes = "1.4.0"
 data-encoding = "2.3.3"
+futures = "0.3.30"
 nix-compat = { path = "../nix-compat" }
 tvix-build = { path = "../build", default-features = false, features = []}
 tvix-eval = { path = "../eval" }