feat: support project_directory for preview action (fixes #49)

[NeilHanlon]

this change introduces several improvements to the Branch Preview github action, aiming to specifically allow users to preview from a subdirectory, as is possible with the main actions. (closes #49 )

Changes

• Add input for project_directory for preview/action.yml, matching the existing actions.
  • Input is optional and defaults to CWD (./)
• Update fastly CLI to latest instead of hardcoded
• add working-directory: ${{ inputs.project_directory }} for all preview action steps
• fix: add FASTLY_API_TOKEN to environment for commands otherwise CLI output ignores --token and warns no profiles exist

[JakeChampion]

This has been released in version 8 of this action 👍 https://github.com/fastly/compute-actions/releases/tag/v8