Replace target_url with log_url and allow user to customize it

[ypresto]

• log_url replaces target_url
• log_url was always defaultUrl, this PR fixes it
  • I'm using this action in pull_request event, but its sha points to Merge [HEAD] to [BASE] commit instead of PR head; /checks URL shows empty result Make checked out ref available to other steps actions/checkout#281 (comment)
  • For workaround, use log_url: https://github.com/${{ github.repository }}/commit/${{ github.event.pull_request.head.sha }}/checks
• Also fixes "View Deployment" button did not appear with example in README.md.

[ypresto]

I have no idea why but I got wrong (merge) commit in "Deployed" link despite of below config specified:

log_url: https://github.com/${{ github.repository }}/commit/${{ github.event.pull_request.head.sha }}/checks

[naquiroz]

Can this be merged?

[chrnorm]

Thankyou @ypresto!