More explicitly name all the dependencies in shell.nix

Add all the relevant tools to compile (only `cargo` needed, `rustc`
is bundled with it in the right version) and develop (`clippy` and
`rustfmt` can then directly be called from cargo; `rust-analyzer`
provides a useful language server for editor).

Made it possible to override nixpkgs by providing an overridable
`pkgs`.

Not sure if the `LD_LIBRARY_PATH` is needed; left it like this for
now.

Signed-off-by: Henner Zeller <h.zeller@acm.org>

hostcontrol/shell.nix

@@ -1,7 +1,22 @@
-with import <nixpkgs> {};
-
+{
+  pkgs ? import <nixpkgs> { },
+}:
 pkgs.mkShell {
-  LD_LIBRARY_PATH = [
+  buildInputs = with pkgs; [
+    cargo
+
+    # Useful tools
+    clippy
+    rustfmt
+    rust-analyzer
+
+    # minimal git
+    git
+    less
+    cacert # For cargo downloads and git to verify certs
+  ];
+  LD_LIBRARY_PATH = with pkgs; [
+    # TODO: needed ?
     "${xorg.libX11}/lib;${xorg.libXcursor}/lib;${xorg.libXi}/lib;${libxkbcommon}/lib"
   ];
 }