v2.3.3: Fix kaos/homebrew-shell failure "installation already attempted"

What's Changed

• build: Use official brew tap for bats-core by @rfay in #48
• build: use main branch for setup-homebrew by @rfay in #47

We started seeing problems like this:

Cloning into '/home/linuxbrew/.linuxbrew/Homebrew/Library/Taps/kaos/homebrew-shell'...
Tapped 7 formulae (25 files, 141.7KB).
Error: Formula installation already attempted: kaos/shell/bats-support

Full Changelog: v2...v2.3.3