nix: add nextest (#5610)

Has some unrelated change that I add the whole .vscode folder to
.gitignore which I also need.

flake.nix

@@ -542,6 +542,7 @@
             ])
             cargo-deny
             rust-analyzer-nightly
+            cargo-nextest
             perl # needed to build vendored OpenSSL
             git-cliff
           ];