feat(users/Profpatsch): set up stow for nix-home

nix-home is (hopefully) gonna be a home-manager alternative for my
home directory.

Files are symlinked into the home directory via GNU stow (since that
is a tried and tested tool), so first step is to set up the base code
for that.

Implements a small tool that reads a single environment variable and
prints it to stdout.

Change-Id: Ifa3fd9f9e1cedc52c3002196d3971b02cb840e80
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4832
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>
Autosubmit: Profpatsch <mail@profpatsch.de>

users/Profpatsch/netstring/default.nix

@@ -57,6 +57,9 @@ let
 
 in depot.nix.readTree.drvTargets {
   inherit
+    toNetstring
+    toNetstringList
+    toNetstringKeyVal
     python-netstring
     rust-netstring
       ;