Try locate readlink first in pyenv-hooks, fix #2654 (#2655)

* Remove all use of `greadlink`, fix #2654

Signed-off-by: Harry Chen <i@harrychen.xyz>

* revert greadlink back in tests

---------

Signed-off-by: Harry Chen <i@harrychen.xyz>
Co-authored-by: Anton Petrov <anton.a.petrov@gmail.com>

plugins/python-build/bin/python-build

@@ -56,9 +56,14 @@ lib() {
 }
 lib "$1"
 
+READLINK=$(type -P readlink)
+if [ -z "$READLINK" ]; then
+  echo "pyenv: cannot find readlink - are you missing GNU coreutils?" >&2
+  exit 1
+fi
 
 resolve_link() {
-  $(type -P greadlink readlink | head -n1) "$1"
+  $READLINK "$1"
 }
 
 abs_dirname() {