ci: use BGL config variables

[willjy1]

Bounty submission for #32.

Summary:

• switch the remaining CI env jobs from BITCOIN_CONFIG to BGL_CONFIG
• keep a BITCOIN_CONFIG fallback in the shared test script for compatibility
• guard the BGL default datadir paths in CI
• enter the BGL-$HOST distdir after make distdir
• use Bitgesell's renamed unit-test binary, BGL-tidy module, and BGL iwyu mapping in the tidy path

Verification:

• git diff --check
• bash -n on the touched shell scripts after LF normalization on Windows