nix-shell regression for shell.nix symlinks to other directories

[knedlsepp]

Describe the bug

I've got a project set up where I have a top-level shell.nix symlink, which points to ./subdirectory/shell.nix.
When I upgraded to the most recent version of nix (2.24.7) suddenly all expressions in ./subdirectory/shell.nix with relative files (such as import ./foo.nix) try to look for these files not in subdirectory but in the top-level directory instead, which then results in a error: path '/home/user/toplevel/foo.nix' does not exist.

Minimal example

I uploaded a reproducer for the issue here: https://github.com/knedlsepp/nix-11630

Additional context

I bisected the issue and the change of behavior was introduced with commit cfe3ee3, part of #11057.

Workaround

Using nix-shell --option nix-shell-always-looks-for-shell-nix false restores the previous behavior. (However the option's name doesn't suggest that this is intended to have an influence)