Add flag --override-input to override specific lock file entries

E.g.

  $ nix flake update ~/Misc/eelco-configurations/hagbard \
    --override-input 'dwarffs/nixpkgs' ../my-nixpkgs

overrides the 'nixpkgs' input of the 'dwarffs' input of the top-level
flake.

Fixes #2837.

src/nix/command.hh

@@ -4,6 +4,7 @@
 #include "args.hh"
 #include "common-eval-args.hh"
 #include "path.hh"
+#include "flake/lockfile.hh"
 
 #include <optional>
 
@@ -42,11 +43,11 @@ struct EvalCommand : virtual StoreCommand, MixEvalArgs
 struct MixFlakeOptions : virtual Args
 {
     bool recreateLockFile = false;
-
     bool saveLockFile = true;
-
     bool useRegistries = true;
 
+    flake::LockFlags lockFlags;
+
     MixFlakeOptions();
 
     flake::LockFileMode getLockFileMode();