manually fix inst/extdata/default.nix, seems git2nix.dev is down

inst/extdata/default.nix

@@ -1,3 +1,20 @@
+# This file was generated by the {rix} R package v0.3.1 on 2023-09-25
+# with following call:
+# >rix(r_ver = "976fa3369d722e76f37c77493d99829540d43845",
+#  > r_pkgs = NULL,
+#  > system_pkgs = NULL,
+#  > git_pkgs = list(package_name = "rix",
+#  > repo_url = "https://github.com/b-rodrigues/rix/",
+#  > branch_name = "master",
+#  > commit = "ae39d2142461688b1be41db800752a949ebb3c7b"),
+#  > ide = "other",
+#  > project_path = "inst/extdata",
+#  > overwrite = TRUE)
+# It uses nixpkgs' revision 976fa3369d722e76f37c77493d99829540d43845 for reproducibility purposes
+# which will install R as it was as of nixpkgs revision: 976fa3369d722e76f37c77493d99829540d43845
+# Report any issues to https://github.com/b-rodrigues/rix
+let
+ pkgs = import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/976fa3369d722e76f37c77493d99829540d43845.tar.gz") {};
   git_archive_pkgs = [(pkgs.rPackages.buildRPackage {
     name = "rix";
     src = pkgs.fetchgit {