AthDataQuality Build Fix, master branch (2021.05.04.)

In preparation for the upcoming move to LCG_100, I made the AthDataQuality build succeed once again.

After some changes in the packages included in AthDataQuality, the build was broken since some time. I did the following to fix it:

• Included CxxUtils (and as its dependency TestTools) in the project.
  • To make these packages build correctly, enabled the XAOD_STANDALONE flag for the build of AthDataQuality.
• Disabled the usage of flake8 during the build, as the required code is not part of AthDataQuality. (I just copied @fwinkl's code from the DetCommon project for this.)
• Removed the explicit setting of the LCG version from AthDataQuality/build.sh, and only kept it in the project's CMakeLists.txt file. This is to avoid unnecessary confusion. (I myself was caught in this while testing the build against LCG_100...)

Since building this project goes relatively quickly, we should really consider adding it to the nightly/CI. Otherwise it is just dead code that we'll continue to forget to keep in a working state... The CI currently will be completely blind to these updates, so it's a bit of a waste to even run it through it...