Absolute paths for overlay files are not determined correctly on Darwin

[alexbrand]

As documented in the script, the script cds into a temp directory during the installation. For this reason, the script needs to compute the absolute path for overlay files (among other files) to be able to reference them during the installation.

The script currently uses stat -f $file to obtain the absolute file. However, this does not work as expected:

$ pwd
/Users/alexbrand/test
$ ls
foo.yaml
$ stat -f foo.yaml
foo.yaml
$ stat -f non-existent-file.yaml
non-existent-file.yaml

This causes issues when trying to use a custom overlay file, as the istioctl install command cannot find the overlay files.