flake: add libb2/lz4/zstd to dev shell

`src/build.rs` probes these via pkg-config when the `static` feature is
enabled, so without them `cargo build --features static` (and therefore
`cargo build --all-features`) fails with "Unable to find libb2" before
any Rust code is compiled.

Note: a fully static build still needs a static libarchive, which the
default nixpkgs `libarchive` does not provide — this change just removes
the libb2/lz4/zstd pkg-config failures, so developers hit a more
meaningful error (or can supply their own static libarchive).

Author: otavio

flake.nix

@@ -37,6 +37,11 @@
             rust-bindgen
             pkg-config
             libarchive
+            # Required by the `static` feature (build.rs probes these via
+            # pkg-config when libarchive is linked statically).
+            libb2
+            lz4
+            zstd
             clang
             llvmPackages.libclang