feat(tvix): add CI targets for checking crate features powerset

Closes: https://b.tvl.fyi/issues/401

With this change all crate features (and their combinations) will be built and
tested in CI.

From now on, when adding/removing a Cargo feature for a crate,
you will want to add it to the features power set that gets tested in CI.
For each crate there's a default.nix with a `mkFeaturePowerset` invocation,
modify the list to include/remove the feature.
Note that you don't want to add "collection" features,
such as `fs` for tvix-[ca]store or `default`.

Change-Id: I966dde1413d057770787da3296cce9c1924570e0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11717
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/castore/Cargo.toml

@@ -92,7 +92,7 @@ rstest_reuse = "0.6.0"
 xattr = "1.3.1"
 
 [features]
-default = []
+default = ["cloud"]
 cloud = [
   "dep:bigtable_rs",
   "object_store/aws",