Default to the Windows system tar, but add an escape hatch

[DilumAluthge]

Fixes #205
Replaces #206 (closes #206)

This is almost identical to #206, except this PR adds a (non-public internal) tar input as an "escape hatch", so that if necessary the user can provide the correct tar to be used.

For example, the user could manually manipulate their PATH before they run this action, and then they could just do this:

- uses: julia-actions/setup-julia@v2
  with:
    version: '1'
    tar: 'tar'

And then this will just pick up the tar that the user put first in their PATH.

[DilumAluthge]

Closed in favor of #206. We can revisit the need for an escape hatch in the future.