fix: add elmerfem channel

When the WSL image is installed, it cannot access the flake inputs
anymore, so Elmer has to be installed via a channel.

flake.nix

@@ -48,6 +48,8 @@
             wsl.elmer.enable = true;
             programs.bash.loginShellInit = "elmer-wsl-welcome";
 
+            nixpkgs.overlays = [ inputs.elmer.overlay ];
+
             # When the config is built from a flake, the NIX_PATH entry of nixpkgs is set to its flake version.
             # Per default the resulting systems aren't flake-enabled, so rebuilds would fail.
             # Note: This does not affect the module being imported into your own flake.
@@ -73,7 +75,6 @@
             system = "x86_64-linux";
             modules = [
               self.nixosModules.default
-              { nixpkgs.overlays = [ inputs.elmer.overlay ]; }
               (config { })
             ];
           };

modules/build-tarball.nix

@@ -12,7 +12,9 @@ let
     # https://github.com/nix-community/NixOS-WSL
 
     { config, lib, pkgs, ... }:
-
+    let
+      elmerfem = import <elmerfem>;
+    in
     {
       imports = [
         # include NixOS-WSL modules
@@ -24,6 +26,8 @@ let
       wsl.elmer.enable = true;
       ${lib.optionalString (!config.wsl.nativeSystemd) "wsl.nativeSystemd = false;"}
 
+      nixpkgs.overlays = [ elmerfem.overlay ];
+
       # 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
@@ -78,8 +82,9 @@ in
           --system ${config.system.build.toplevel} \
           --substituters ""
 
-        echo "[Elmer-WSL] Adding channel..."
+        echo "[Elmer-WSL] Adding channels..."
         nixos-enter --root "$root" --command 'HOME=/root nix-channel --add https://github.com/ElmerCSC/Elmer-WSL/archive/refs/heads/main.tar.gz elmer-wsl'
+        nixos-enter --root "$root" --command 'HOME=/root nix-channel --add https://github.com/ElmerCSC/elmerfem/archive/refs/heads/devel.tar.gz elmerfem'
 
         echo "[Elmer-WSL] Adding default config..."
         ${if cfg.configPath == null then ''