nixos: allow overriding labels with environment variables

This reintroduces a better version of what 2a05368ff3 removed.

nixos/modules/misc/label.nix

@@ -18,6 +18,24 @@ in
 
         If you ever wanted to influence the labels in your GRUB menu,
         this is the option for you.
+
+        The default is <option>system.nixos.tags</option> separated by
+        "-" + "-" + <envar>NIXOS_LABEL_VERSION</envar> environment
+        variable (defaults to the value of
+        <option>system.nixos.version</option>).
+
+        Can be overriden by setting <envar>NIXOS_LABEL</envar>.
+
+        Useful for not loosing track of configurations built from different
+        nixos branches/revisions, e.g.:
+
+        <screen>
+        #!/bin/sh
+        today=`date +%Y%m%d`
+        branch=`(cd nixpkgs ; git branch 2>/dev/null | sed -n '/^\* / { s|^\* ||; p; }')`
+        revision=`(cd nixpkgs ; git rev-parse HEAD)`
+        export NIXOS_LABEL_VERSION="$today.$branch-''${revision:0:7}"
+        nixos-rebuild switch</screen>
       '';
     };
 
@@ -46,7 +64,9 @@ in
   config = {
     # This is set here rather than up there so that changing it would
     # not rebuild the manual
-    system.nixos.label = mkDefault (concatStringsSep "-" (sort (x: y: x < y) cfg.tags) + "-" + cfg.version);
+    system.nixos.label = mkDefault (maybeEnv "NIXOS_LABEL"
+                                             (concatStringsSep "-" (sort (x: y: x < y) cfg.tags)
+                                             + "-" + maybeEnv "NIXOS_LABEL_VERSION" cfg.version));
   };
 
 }