change ${{ github.action_path }} with $GITHUB_ACTION_PATH

[carlosala]

Hi! Super great action, kudos for all the work!

We noticed that, when using this action inside a custom container, ${{ github.action_path }} is not properly set up. Instead, $GITHUB_ACTION_PATH is set up as expected (it's already being used in this action, there should not be difference). Github seem to be tracking this issue actions/runner#2185 but they didn't merged the fix yet.
This change should not change the functionality for anyone.

Thanks @2bndy5 and all the team for the hard work!

[2bndy5]

At first, I was worried about windows not recognizing the unix bash style syntax. But then I remembered that these cmds are all executed in bash.

[carlosala]

That's it! Actually it's sh as far as I remember. But, yes! Always POSIX shells :)
Thanks!

[carlosala]

Thanks for the fast merge :)
Could we get it tagged into the v2 version? Thanks!

[shenxianpeng]

The tests look good so I have tagged into the v2 version.