docs(nixery): dynamically display current nixpkgs commit

People occasionally ask what the current nixpkgs commit is on
nixery.dev (see e.g. https://github.com/tazjin/nixery/issues/153).

With this change, the commit is displayed on nixery.dev if Nixery is
built for the TVL deployment.

Change-Id: I795220214db5a367a126c9b4bd03754e9f144940
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5881
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>

tools/nixery/docs/default.nix

@@ -7,7 +7,7 @@
 # Some of the documentation is pulled in and included from other
 # sources.
 
-{ fetchFromGitHub, mdbook, runCommand, rustPlatform }:
+{ fetchFromGitHub, mdbook, runCommand, rustPlatform, postamble ? "" }:
 
 let
   nix-1p = fetchFromGitHub {
@@ -17,10 +17,14 @@ let
     sha256 = "1pf9i90gn98vz67h296w5lnwhssk62dc6pij983dff42dbci7lhj";
   };
 in
-runCommand "nixery-book" { } ''
+runCommand "nixery-book"
+{
+  POSTAMBLE = postamble;
+} ''
   mkdir -p $out
   cp -r ${./.}/* .
   chmod -R a+w src
   cp ${nix-1p}/README.md src/nix-1p.md
+  echo "''${POSTAMBLE}" >> src/nixery.md
   ${mdbook}/bin/mdbook build -d $out
 ''