electrs: 0.7.1 -> 0.8.3

Use buildRustPackage instead of buildRustCrate (via crate2nix).

buildRustPackage builds the whole executable and its libraries in a
single `cargo build` process.

With the create2nix approach each library is built in a separate derivation,
directly using rustc instead of the cargo wrapper.

Benefits of buildRustPackage:
- Much simpler to maintain
- Package derivation evaluates much faster

Benefits of crate2nix:
- Build can be distributed over multiple build hosts
- Better sharing of common dependencies between different builds
- More fine-grained rebuilding on build failures

In nixpkgs buildRustPackage is used for almost all Rust pkgs, it's
also a better fit for our use case.

pkgs/default.nix

@@ -4,7 +4,7 @@
   lightning-charge = pkgs.callPackage ./lightning-charge { };
   nanopos = pkgs.callPackage ./nanopos { };
   spark-wallet = pkgs.callPackage ./spark-wallet { };
-  electrs = (pkgs.callPackage ./electrs { }).rootCrate.build;
+  electrs = pkgs.callPackage ./electrs { };
   elementsd = pkgs.callPackage ./elementsd { withGui = false; };
   hwi = pkgs.callPackage ./hwi { };
   pylightning = pkgs.python3Packages.callPackage ./pylightning { };