[CI][Crossbow] NIghtly fuzzit build broken in docker-compose refactor

[asfimport]

See https://circleci.com/gh/ursa-labs/crossbow/4959 for example.

/arrow/ci/scripts/fuzzit_build.sh: line 26: pushd: /arrow/cpp/build/relwithdebinfo: No such file or directory

Scrolling up in the logs, it looks like the build dir is actually /build/cpp, so given that, we should pushd /build/cpp/relwithdebinfo.

cc @kszucs

Reporter: Neal Richardson / @nealrichardson

Related issues:

• [CI][Crossbow] Fix or delete fuzzit jobs (is superceded by)

_{Note: This issue was originally created as ARROW-7182. Please see the migration documentation for further details.}