Add --update-input flag to update a specific flake input

Typical usage:

  $ nix flake update ~/Misc/eelco-configurations/hagbard --update-input nixpkgs

to update the 'nixpkgs' input of a flake while leaving every other
input unchanged.

The argument is an input path, so you can do e.g. '--update-input
dwarffs/nixpkgs' to update an input of an input.

Fixes #2928.

src/libexpr/flake/flake.hh

@@ -78,7 +78,12 @@ struct LockFlags
        allowed. */
     bool allowMutable = true;
 
+    /* Flake inputs to be overriden. */
     std::map<InputPath, FlakeRef> inputOverrides;
+
+    /* Flake inputs to be updated. This means that any existing lock
+       for those inputs will be ignored. */
+    std::set<InputPath> inputUpdates;
 };
 
 LockedFlake lockFlake(