feat: add support for custom rust toolchains compatible with rustup

rust/default.nix

@@ -25,10 +25,17 @@
       }: let
         cfg = config.krantz.rust;
 
-        craneLib = inputs.crane.mkLib pkgs;
         src = cfg.srcFiltered;
         manifest = (lib.importTOML "${src}/Cargo.toml").package;
 
+        craneLib =
+          if builtins.pathExists cfg.srcToolchain
+          then
+            (inputs.crane.mkLib (pkgs.extend inputs.rust-overlay.overlays.default)).overrideToolchain (
+              p: p.rust-bin.fromRustupToolchainFile cfg.srcToolchain
+            )
+          else inputs.crane.mkLib pkgs;
+
         buildInputs = cfg.runtimeDeps;
 
         # build dependencies
@@ -55,6 +62,13 @@
             defaultText = lib.literalExpression "config.krantz.rust.craneLib cfg.src";
             example = lib.literalExpression "./.";
           };
+          srcToolchain = lib.mkOption {
+            description = "A path to the rust-toolchain(.toml) file for the repo. Will use the toolchain from nixpkgs if the file does not exist.";
+            type = lib.types.path;
+            default = "${cfg.src}/rust-toolchain.toml";
+            defaultText = lib.literalExpression "cfg.src + \"/rust-toolchain.toml\"";
+            example = lib.literalExpression "./rust-toolchain";
+          };
 
           runtimeDeps = lib.mkOption {
             description = "Packages needed for the app to run.";