vscode: Fix url handler MimeType for vscode-insiders

According to upstream RPMs, while using vscode-insiders, the URL handler MimeType should be
`x-scheme-handler/vscode-insiders` instead of `x-scheme-handler/vscode`.

See pull request 293460 and pull request 288425.

pkgs/applications/editors/vscode/generic.nix

@@ -150,7 +150,7 @@ in
     icon = "vs${executableName}";
     startupNotify = true;
     categories = [ "Utility" "TextEditor" "Development" "IDE" ];
-    mimeTypes = [ "x-scheme-handler/vscode" ];
+    mimeTypes = [ "x-scheme-handler/vs${executableName}" ];
     keywords = [ "vscode" ];
     noDisplay = true;
   };