Delay nix-gc until /home is mounted

Prevents starting the garbage collector before the remote FS are
mounted, in particular /home. Otherwise, all the gcroots which have
symlinks in /home will be considered stale and they will be removed.

See: #79
Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>

m/common/base/nix.nix

@@ -32,6 +32,21 @@
     };
   };
 
+  # The nix-gc.service can begin its execution *before* /home is mounted,
+  # causing it to remove all gcroots considering them as stale, as it cannot
+  # access the symlink. To prevent this problem, we force the service to wait
+  # until /home is mounted as well as other remote FS like /ceph.
+  systemd.services.nix-gc = {
+    # Start remote-fs.target if not already being started and fail if it fails
+    # to start. It will also be stopped if the remote-fs.target fails after
+    # starting successfully.
+    bindsTo = [ "remote-fs.target" ];
+    # Wait until remote-fs.target fully starts before starting this one.
+    after = [ "remote-fs.target"];
+    # Ensure we can access a remote path inside /home
+    unitConfig.ConditionPathExists = "/home/Computational";
+  };
+
   # This value determines the NixOS release from which the default
   # settings for stateful data, like file locations and database versions
   # on your system were taken. It‘s perfectly fine and recommended to leave