[gha][lbt] calculate PR number earlier for slack log
[gha][lbt] calculate PR number earlier for slack log

* Changes PR number and details calculation, and reorders initial
steps
* fetch base ref for use in compat test
* New button in Slack to Visit PR
* Slack message uses the report.json

Closes: #5701

aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,libra/libra