Add custom nix-daemon exporter

Allows us to see which derivations are being built in realtime. It is a
bit of a hack, but it seems to work. We simply look at the environment
of the child processes of nix-daemon (usually bash) and then look for
the $name variable which should hold the current derivation being
built. Needs root to be able to read the environ file of the different
nix-daemon processes as they are owned by the nixbld* users.

See: https://discourse.nixos.org/t/query-ongoing-builds/23486

m/hut/nix-daemon-builds.sh

@@ -18,7 +18,7 @@ Content-Type: text/plain; version=0.0.4; charset=utf-8; escaping=values
 EOF
 
 for pid in $pids2; do
-  name=$(cat /proc/$pid/environ 2>/dev/null | tr '\0' '\n' | rg "^name=(.+)" - --replace '$1' | tr -dc ' [:alnum:]_\-\.')
+  name=$(cat /proc/$pid/environ 2>/dev/null | tr '\0' '\n' | rg "^name=(.+)" - --replace '$1')
   user=$(ps -o uname= -p "$pid")
   if [ -n "$name" -a -n "$user" ]; then
     printf 'nix_daemon_build{user="%s",name="%s"} 1\n' "$user" "$name"