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

[MyTH-zyxeon]

Review assist for maintainers:

This PR is a reasonable CI rename cleanup for the Bitgesell fork: it moves remaining CI configuration knobs from BITCOIN_CONFIG/BITCOIN_CONFIG_ALL toward BGL_CONFIG/BGL_CONFIG_ALL, updates the default datadir guard, and switches distdir/test/tidy paths to BGL names.

Suggested merge checks:

• Run bash -n ci/test/03_test_script.sh ci/test/00_setup_env_arm.sh ci/test/00_setup_env_mac_cross.sh ci/test/00_setup_env_native_tidy.sh ci/test/00_setup_env_native_tsan.sh after normalizing line endings.
• Exercise at least one CI job or local dry run with only BGL_CONFIG set, and one compatibility dry run with only legacy BITCOIN_CONFIG set, to confirm the fallback BGL_CONFIG=${BGL_CONFIG:-${BITCOIN_CONFIG:-}} behaves as intended.
• Confirm make distdir VERSION="$HOST" really emits BGL-$HOST in this fork; if it still emits an upstream-style directory, the cd "${BASE_BUILD_DIR}/BGL-$HOST" line will break CI.
• Confirm renamed build artifacts exist for test_BGL, contrib/devtools/BGL-tidy, BGL-tidy-tests, libBGL-tidy.so, and contrib/devtools/iwyu/BGL.core.imp before merging the tidy path changes.
• Recheck the default datadir guard names on macOS and Linux; the case-sensitive BGL / .BGL paths should match the actual wallet/node defaults used by this fork.

I do not see consensus, wallet/key, signing, RPC runtime, or live-chain behavior changes in this patch as written. The main merge risk is CI breakage if any renamed distdir or tidy artifact has not actually been renamed in the build system yet.