nix: simplify devShell and envrc

There is no need for the ';' in the `.envrc` file.

Do not run checks in devShell so that it's possible to enter the shell
while code is being edited and is not compiling. Also include
`cargo-nextest` so that one can test in the shell.

Signed-off-by: Fintan Halpenny <Fintan Halpenny@gmail.com>
X-Clacks-Overhead: GNU Terry Pratchett

.envrc

@@ -1 +1 @@
-use flake;
+use flake

flake.nix

@@ -213,12 +213,10 @@
       };
 
       devShells.default = craneLib.devShell {
-        # Inherit inputs from checks.
-        checks = self.checks.${system};
-
         # Extra inputs can be added here; cargo and rustc are provided by default.
         packages = [
           pkgs.cargo-watch
+          pkgs.cargo-nextest
           pkgs.ripgrep
           pkgs.rust-analyzer
         ];