Change the pihole mount directories

Whilst doing this to /mnt/media, the adlist was showing as -2 and saying
that DNS resolution was not possible. This was preventing any ads from
being blocked.

Moving this to a local directory seems to fix the issue and I'll
investigate further at a later date.

nix/modules/nixos/features/homelab/pi-hole.nix

@@ -31,8 +31,8 @@ in
           };
 
           volumes = [
-            "/mnt/media/pihole/etc-dnsmasq.d:/etc/dnsmasq.d:rw"
-            "/mnt/media/pihole/etc-pihole:/etc/pihole:rw"
+            "/home/opdavies/pihole/etc-dnsmasq.d:/etc/dnsmasq.d:rw"
+            "/home/opdavies/pihole/etc-pihole:/etc/pihole:rw"
           ];
 
           ports = [