fae49f6e424f31e93c5620d5ff893fb517ef4a8b ci: Add and document BASE_BUILD_DIR (MarcoFalke)
Pull request description:
Also fixes#18768
ACKs for top commit:
hebasto:
re-ACK fae49f6e424f31e93c5620d5ff893fb517ef4a8b, which is essentially the same as the previously [reviewed changes](https://github.com/bitcoin/bitcoin/pull/18735#pullrequestreview-400581536).
Tree-SHA512: 216565a05ccd513dd9f114b2333d3c283fd71914d32f9b05f145cb7c70633b083ff8ef60798d6f22f4be6a4d652b03806551fd74b5b596c92968501a4d9726d2