fix: nix flake check also builds the package

I mentioned that nix flake check would also build the default package. This is not the case as I found out today reading the mane page of `nix flake check` more carefully.
In order to also build the default package It has to be added to the checks. **Or** nix build should be added back to the workflow.

Signed-off-by: naŭ glenda <plan9git@proton.me>

flake.nix

@@ -153,6 +153,7 @@
         checks = {
           formatting = treefmtEval.config.build.check self;
           build = packages.check;
+          default = packages.default;
           test = packages.test;
           lint = packages.clippy;
           trycmd = packages.trycmd;