Update Nix backend to use Nix Flakes

backends/nix/buildenv.nix.gen.hh

@@ -0,0 +1,27 @@
+R"foo(
+{ derivations, manifest }:
+
+derivation {
+  name = "user-environment";
+  system = "builtin";
+  builder = "builtin:buildenv";
+
+  inherit manifest;
+
+  # !!! grmbl, need structured data for passing this in a clean way.
+  derivations =
+    map (d:
+      [ (d.meta.active or "true")
+        (d.meta.priority or 5)
+        (builtins.length d.outputs)
+      ] ++ map (output: builtins.getAttr output d) d.outputs)
+      derivations;
+
+  # Building user environments remotely just causes huge amounts of
+  # network traffic, so don't do that.
+  preferLocalBuild = true;
+
+  # Also don't bother substituting.
+  allowSubstitutes = false;
+}
+)foo"

backends/nix/meson.build

@@ -1,23 +1,24 @@
 add_languages('cpp')
 
-nix_expr_dep = dependency('nix-expr', version: '>=1.12')
-nix_main_dep = dependency('nix-main', version: '>=1.12')
-nix_store_dep = dependency('nix-store', version: '>=1.12')
+nix_expr_dep = dependency('nix-expr', version: '>=2.6')
+nix_main_dep = dependency('nix-main', version: '>=2.6')
+nix_store_dep = dependency('nix-store', version: '>=2.6')
+nix_cmd_dep = dependency('nix-cmd', version: '>=2.6')
 
 shared_module(
   'pk_backend_nix',
   'pk-backend-nix.cc',
-  'nix-helpers.cc',
   'nix-lib-plus.cc',
   include_directories: packagekit_src_include,
   dependencies: [
     packagekit_glib2_dep,
     nix_expr_dep,
     nix_main_dep,
+    nix_cmd_dep,
     nix_store_dep,
     gmodule_dep,
   ],
-  c_args: [
+  cpp_args: [
     '-DPK_COMPILATION=1',
     '-DG_LOG_DOMAIN="PackageKit-Nix"',
   ],