chore: update old (unsupported) version of nix used in script

Author: jfroche

nix/packages/sync-exts-versions.nix

@@ -11,7 +11,7 @@ runCommand "sync-exts-versions" { } ''
     --subst-var-by 'YQ' '${yq}/bin/yq' \
     --subst-var-by 'JQ' '${jq}/bin/jq' \
     --subst-var-by 'NIX_EDITOR' '${nix-editor.packages.nix-editor}/bin/nix-editor' \
-    --subst-var-by 'NIXPREFETCHURL' '${nixVersions.nix_2_20}/bin/nix-prefetch-url' \
-    --subst-var-by 'NIX' '${nixVersions.nix_2_20}/bin/nix'
+    --subst-var-by 'NIXPREFETCHURL' '${nixVersions.nix_2_29}/bin/nix-prefetch-url' \
+    --subst-var-by 'NIX' '${nixVersions.nix_2_29}/bin/nix'
   chmod +x $out/bin/sync-exts-versions
 ''