flake: More concise devShell

No need for C compiler in development shell + use "packages" to
explicitly refer to devShell packages

flake.nix

@@ -51,20 +51,20 @@
     in {
       packages.x86_64-linux = {
         inherit thermostat;
+        default = thermostat;
       };
 
       hydraJobs = {
         inherit thermostat;
       };
 
-      devShell.x86_64-linux = pkgs.mkShell {
+      devShells.x86_64-linux.default = pkgs.mkShellNoCC {
         name = "thermostat-dev-shell";
-        buildInputs = with pkgs; [
+        packages = with pkgs; [
           rust openocd dfu-util
           ] ++ (with python3Packages; [
             numpy matplotlib
           ]);
       };
-      defaultPackage.x86_64-linux = thermostat;
     };
 }