When wanting to access the Filesystem through GitHub Actions, the main location that is focused is the root of the repository when a workflow is being run. To get to that location, GitHub Provides a GITHUB_WORKSPACE environment variable that is automatically setup as long as you've run the actions/checkout action.

For instance, if we're in a repo and want to grab all the files in a content/posts/ directory, here's a node snippet to access such:

const fs = require('fs')
const path = require('path')

const posts = fs.readdir(
  path.join(process.env.GITHUB_WORKSPACE, 'content', 'posts')
)

With this, whether an action is local within the repo or a 3rd party action, either will be able to navigate to your repo within a workflow run and access its contents.