Adapt for Enterprise version - respect GITHUB ENV variables

[sielaq]

Hi,

this tiny code makes this action fully compatible with Enterprise version,
to respect ENV variables such as GITHUB_SERVER_URL and additionally GITHUB_API_URL
which are used by Github Enterprise runners.

[trstringer]

Thank you for the contribution, @sielaq! Before I merge this in, can you confirm that you've tested this in a GitHub Enterprise environment?

[sielaq]

Hi @trstringer ,
yes, I can confirm that
this is why I created that PR - to use your action in our Enterprise Github workflow :)

[trstringer]

Great! Thanks for the contribution!!!