importNpmLock: fix check for .bin in linkNodeModulesHook

node_modules/.bin/ is expected to be (symlink to) a directory rather
than a regular file, so we check for that.

This seems to work for pre-existing directories as well as those
we link to the store (managed & unmanaged in the hooks terminology).

I am not entirely sure whether the original check was simply a typo
or intentional for some use-cases.

pkgs/build-support/node/import-npm-lock/hooks/link-node-modules-hook.sh

···
     fi
 
     @nodejs@ @script@ @storePrefix@ "${npmDeps}/node_modules"
-    if test -f node_modules/.bin; then
+    if test -d node_modules/.bin; then
         export PATH=$(readlink -f node_modules/.bin):$PATH
     fi