fix: default actor

[ninjaprox]

This PR is to unset the default value of actor input, otherwise INPUT_ACTOR will be always "GITHUB_ACTOR" instead of GITHUB_ACTOR env var if the input is omitted.

[OleksiyRudenko]

@ninjaprox Thank you for noticing this and suggesting a fix. A stupid mistake from my side.
Now, I am thinking of setting the default value as ${{ github.actor }}, which effectively dubs GITHUB_ACTOR env variable. Having this defined in action.yml seems to be more obvious and consistent.
What do you think?

[ninjaprox]

@OleksiyRudenko I'm not sure if ${{ github.actor }} works in action.yml, expressions seem to work only in workflow files. Mentioning it in the description should suffice for users.