Restore input substitution

The ability to substitute inputs was removed in #10612 because it was
broken: with user-specified inputs containing a `narHash` attribute,
substitution resulted in an input that lacked the attributes returned
by the real fetcher (such as `lastModified`).

To fix this, we introduce a new input attribute `final`. If `final =
true`, fetching the input cannot add or change any attributes.

We only attempt to substitute inputs that have `final = true`. This is
implied by lock file entries; we only write a lock file if all its
entries are "final".

The user can specified `final = true` in `fetchTree`, in which case it
is their responsibility to ensure that all attributes returned by the
fetcher are included in the `fetchTree` call. For example,

  nix eval --impure --expr 'builtins.fetchTree { type = "github"; owner = "NixOS"; repo = "patchelf"; final = true; narHash = "sha256-FSoxTcRZMGHNJh8dNtKOkcUtjhmhU6yQXcZZfUPLhQM="; }'

succeeds in a store path with the specified NAR hash exists or is
substitutable, but fails with

  error: fetching final input '{"final":true,"narHash":"sha256-FSoxTcRZMGHNJh8dNtKOkcUtjhmhU6yQXcZZfUPLhQM=","owner":"NixOS","repo":"patchelf","type":"github"}' resulted in different input '{"final":true,"lastModified":1718457448,"narHash":"sha256-FSoxTcRZMGHNJh8dNtKOkcUtjhmhU6yQXcZZfUPLhQM=","owner":"NixOS","repo":"patchelf","rev":"a0f54334df36770b335c051e540ba40afcbf8378","type":"github"}'

src/libexpr/call-flake.nix

@@ -44,7 +44,8 @@ let
               overrides.${key}.sourceInfo
             else
               # FIXME: remove obsolete node.info.
-              fetchTree (node.info or {} // removeAttrs node.locked ["dir"]);
+              # Note: lock file entries are always final.
+              fetchTree (node.info or {} // removeAttrs node.locked ["dir"] // { final = true; });
 
           subdir = overrides.${key}.dir or node.locked.dir or "";