Allow missing flake.nix for --override-input target

At this point, we don’t know if the input is a flake or not. So, we
should allow the user to override the input with a directory without a
flake.nix.

Ideally, we could figure whether the input was originally a flake or
not, but that would require instantiating the whole flake. So just
allow it to be missing here, and rely on checks later on to verify the
input for us.

src/libcmd/installables.cc

@@ -97,7 +97,7 @@ MixFlakeOptions::MixFlakeOptions()
             lockFlags.writeLockFile = false;
             lockFlags.inputOverrides.insert_or_assign(
                 flake::parseInputPath(inputPath),
-                parseFlakeRef(flakeRef, absPath(".")));
+                parseFlakeRef(flakeRef, absPath("."), true));
         }}
     });