Merge #17900: ci: Combine 32-bit build with CentOS 7 build

ef63f5fc1136ad2a2cd080d44142a2ee3945c238 ci: Combine 32-bit build with CentOS 7 build (Sebastian Falbesoner)

Pull request description:

  Combines the CentOS build with the 32-bit (i686) build to avoid Travis bottlenecks, as suggested in #17757 by MarcoFalke. This keeps most of the properties of the 32-bit build (dash as config shell, building QT5 GUI) and just builds it with depends inside the CentOS docker container.

  Making the depends in `05_before_script.sh` with unset config shell (`CONFIG_SHELL=`)

  https://github.com/bitcoin/bitcoin/blob/6196e930018181301b5972842ae384ea4288ff34/ci/test/05_before_script.sh#L28

  caused problems for building the library libevent (resulting in a Makefile with no shell set (`SHELL=`)), that's why I set it explicitely to `/bin/bash` if we have a CentOS Docker container.

  A Travis output of this 32-bit CentOS build can be seen here: https://travis-ci.org/theStack/bitcoin/jobs/634472394 (has been restarted once due to too long build time and appearance of the `CACHE_ERR_MSG`).

  For anyone wanting to verify the outputs, I found these instructions useful to reproduce a Travis build locally: https://github.com/erdc/proteus/wiki/Replicating-the-TravisCI-Environment-on-your-Local-Machine (steps 1-3). In this case it's a bit tricky since you run Docker inside Docker -- within the Travis Docker container, the CentOS Docker container is created. To make this possible, the Docker socket has to be exposed to the Travis container via bind-mounting (`docker run -v /var/run/docker.sock:/var/run/docker.sock ...`), as suggested in https://stackoverflow.com/a/33003273.

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Tree-SHA512: af508241cec3a10a66c37673d56691717b78375340e910fcdd3fb3870741eba623a436e1e85b26b54f013375611896f5411c5a7fec2437d367d27172230129fe

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