buildBazelPackage: fix directory symlink handling
buildBazelPackage: fix directory symlink handling

The previous behaviour would work fine as long as `symlink` is a link to
a file. If is a link to a directory though, the new `ln` wouldn't
overwrite it but would create a new link *in that directory* (with the
name of the link source).

Instead, we can precompute the target location, then first remove the
symlink and write the new one in its place.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs