nix: Change postgrest-nixpkgs-upgrade to unstable

Since we're currently on the unstable channel and will likely stay there
for a while, let's encode this in the update script.

Once we switch back to stable, if we do, we can still adjust it again.

nix/tools/nixpkgsTools.nix

@@ -33,7 +33,7 @@ let
       ''
         # The list of refs is sorted. The first result will be nixpkgs-unstable, the second the latest stable branch.
         # shellcheck disable=SC2207
-        commit=($(${curl}/bin/curl "${refUrl}" -H "${githubV3Header}" | ${jq}/bin/jq -r 'sort_by(.ref) | reverse | [.[1].object.sha, .[1].ref] | @tsv'))
+        commit=($(${curl}/bin/curl "${refUrl}" -H "${githubV3Header}" | ${jq}/bin/jq -r 'sort_by(.ref) | reverse | [.[0].object.sha, .[0].ref] | @tsv'))
         tarballUrl="${tarballUrlBase}''${commit[0]}.tar.gz"
         tarballHash="$(${nix}/bin/nix-prefetch-url --unpack "$tarballUrl")"
         currentDate="$(${coreutils}/bin/date --iso)"