ci: fix slackify of markdown code

[derberg]

• reverting changes from https://github.com/asyncapi/.github/pull/235/files?diff=split&w=0
• reading markdown input by passing it to environment variable first

Tested:

• slack action example https://github.com/derberg/test-experiment/blob/master/.github/actions/slackify-markdown/action.yml
• workflow example https://github.com/derberg/test-experiment/blob/master/.github/workflows/testtest.yml
• comment in issue example https://github.com/derberg/test-experiment/issues/150#issuecomment-1540116446
• run green example https://github.com/derberg/test-experiment/actions/runs/4926234621/jobs/8801502707

cc @akshatnema

fixes https://github.com/asyncapi/.github/issues/231

[quetzalliwrites]

I'm not confident merging this until another person also gives it a look over, like @akshatnema 😅

[derberg]

/rtm