bootstrap: override flake inputs if needed

modules/build/initial-build.nix

@@ -4,6 +4,17 @@
 
 with lib;
 
+let
+  defaultNixpkgsBranch = "nixos-22.05";
+  defaultNixOnDroidBranch = "release-22.05";
+
+  defaultNixpkgsChannel = "https://nixos.org/channels/${defaultNixpkgsBranch}";
+  defaultNixOnDroidChannel = "https://github.com/t184256/nix-on-droid/archive/${defaultNixOnDroidBranch}.tar.gz";
+
+  defaultNixpkgsFlake = "github:NixOS/nixpkgs/${defaultNixpkgsBranch}";
+  defaultNixOnDroidFlake = "github:t184256/nix-on-droid/${defaultNixOnDroidBranch}";
+in
+
 {
 
   ###### interface
@@ -14,23 +25,35 @@ with lib;
       channel = {
         nixpkgs = mkOption {
           type = types.str;
-          default = "https://nixos.org/channels/nixos-22.05";
+          default = defaultNixpkgsChannel;
           description = "Channel URL for nixpkgs.";
         };
 
         nix-on-droid = mkOption {
           type = types.str;
-          default = "https://github.com/t184256/nix-on-droid/archive/release-22.05.tar.gz";
+          default = defaultNixOnDroidChannel;
           description = "Channel URL for nix-on-droid.";
         };
       };
 
       flake = {
+        nixpkgs = mkOption {
+          type = types.str;
+          default = defaultNixpkgsFlake;
+          description = "Flake URL for nixpkgs.";
+        };
+
         nix-on-droid = mkOption {
           type = types.str;
-          default = "github:t184256/nix-on-droid/release-22.05";
+          default = defaultNixOnDroidFlake;
           description = "Flake URL for nix-on-droid.";
         };
+
+        inputOverrides = mkEnableOption "" // {
+          description = ''
+            Whether to override the standard input URLs in the initial <filename>flake.nix</filename>.
+          '';
+        };
       };
     };
 
@@ -41,7 +64,13 @@ with lib;
 
   config = {
 
-    build.initialBuild = true;
+    build = {
+      initialBuild = true;
+
+      flake.inputOverrides =
+        config.build.flake.nixpkgs != defaultNixpkgsFlake
+        || config.build.flake.nix-on-droid != defaultNixOnDroidFlake;
+    };
 
     # /etc/group and /etc/passwd need to be build on target machine because
     # uid and gid need to be determined.

modules/environment/login/login-inner.nix

@@ -78,6 +78,15 @@ writeText "login-inner" ''
         echo "Installing flake from default template..."
         ${nixCmd} flake new ${config.user.home}/.config/nix-on-droid --template ${config.build.flake.nix-on-droid}
 
+        ${lib.optionalString config.build.flake.inputOverrides ''
+          echo "Overriding input urls in flake..."
+          ${nixCmd} run nixpkgs#gnused -- \
+            -i \
+            -e 's,\"github:NixOS/nixpkgs.*\",\"${config.build.flake.nixpkgs}\",' \
+            -e 's,\"github:t184256/nix-on-droid.*\",\"${config.build.flake.nix-on-droid}\",' \
+            "${config.user.home}/.config/nix-on-droid/flake.nix"
+        ''}
+
         echo "Installing first nix-on-droid generation..."
         ${nixCmd} run ${config.build.flake.nix-on-droid} -- switch --flake ${config.user.home}/.config/nix-on-droid#deviceName