feat(users/sterni/emacs): provide BQN interpreter via Nix

For this we create a directory containing a nix-inject.el file using
writeTextFile where we can string interpolate as much as we please and
merge that into a single emacs.d directory with the config *.el files
tracked in the normal tree using symlinkJoin.

Change-Id: I0e39591587a54527214783d4380456d2763da091
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4324
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: sterni <sternenseemann@systemli.org>

users/sterni/emacs/init.el

@@ -180,5 +180,6 @@
   :mode (("\\.adoc\\'" . adoc-mode)))
 
 (require 'subscriptions)
+(require 'nix-inject)
 
 (provide 'init)