feat(tools/depot-nixpkgs-update): tool to update our pin

Very simplistic tool to aid with updating our nixpkgs channel pins:
This tool prints the correct two `*Hashes` sets to stdout, so you can
easily delete the appropriate lines in `third_party/nixpkgs/default.nix`,
run

    ./bin/depot-nixpkgs-update | wl-copy

and paste it into your editor.

Doing this fully automatically would be possible, but would either
a) require changing `default.nix`, so it is regex-able more easily,
b) touching something like rnix-parser which I have no motivation to
at the moment or
c) searching for the old hashes and replacing them with the new
ones. This may be a simple and worthwhile improvement in the future.

Change-Id: I4df44e3827ce9ff6e4fe2d336c08016d799e21a7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3252
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

tools/depot-nixpkgs-update.nix

@@ -0,0 +1,44 @@
+{ pkgs, depot, ... }:
+
+let
+  inherit (depot.nix)
+    getBins
+    ;
+
+  stableRelease = "21.05";
+
+  channelsUrl = "https://channels.nixos.org";
+  archiveUrl = "https://github.com/NixOS/nixpkgs/archive/";
+
+  bins = getBins pkgs.nix [ "nix-prefetch-url" ]
+    //   getBins pkgs.curl [ "curl" ]
+    ;
+
+in
+
+pkgs.writers.writeDashBin "depot-nixpkgs-update" ''
+  set -e
+
+  printSet() {
+    setname="$1"
+    shift
+    channel="$1"
+    shift
+
+    commit="$(${bins.curl} -L "${channelsUrl}/$channel/git-revision")"
+    date="$(curl -i -L "${channelsUrl}/$channel/git-revision" \
+      | grep ^last-modified \
+      | sed 's/^last-modified: \(.\+\)\r/\1/')"
+    hash="$(${bins.nix-prefetch-url} --unpack --type sha256 "${archiveUrl}/$commit.tar.gz")"
+
+    printf '%s\n' "
+    # Tracking $channel as of $(date --rfc-3339=date --date="$date").
+    $setname = {
+      commit = \"$commit\";
+      sha256 = \"$hash\";
+    };"
+  }
+
+  printSet unstableHashes nixos-unstable
+  printSet stableHashes nixos-${stableRelease}
+''