disko-install: resolve symlinks in mount point path

Nix refuses to operate when the store path or its parent directories
contain symlinks. On some systems /mnt can be a symlink, causing
disko-install to fail with:

  error: the path '"/mnt"' is a symlink; this is not allowed for the
  Nix store and its parent directories

Use realpath to resolve symlinks before passing the mount point to
nix-store and nixos-install commands.

disko-install

@@ -224,6 +224,9 @@ main() {
 
   maybeRun mkdir -p "${mountPoint}"
   maybeRun chmod 755 "${mountPoint}" # bcachefs wants 755
+  # Resolve symlinks in mountPoint path to satisfy nix-store requirements
+  # (nix refuses symlinks in store path hierarchy)
+  mountPoint=$(realpath "${mountPoint}")
   # shellcheck disable=SC2064
   if [[ -z ${dry_run-} ]]; then
      trap cleanupMountPoint EXIT