Github Actions: many minor updates

+ Add Ubuntu-20.04 Github Actions runner
+ Use as many system libraries as possible on all
  platforms except Windows
+ Checkout the git repository into a directory named "source",
  so that it's possible to checkout more repositories in the future
+ Configure with install prefix set to a temporary path,
  it will be used for uploading binary artifacts in the future
+ Configure with -pkg-config instead of -feature-pkg-config;
  this was the traditional way and it works for cmake now
+ Split long lines using double quotes instead of backslashes,
  makes the yaml file more readable and does not leak
  backslashes into the command line
+ Adjust cache to work in multiple branches, provided that the
  environment variables at the top of the file are set correctly
+ Do not let homebrew waste time cleaning up
+ Print logfiles in case of failure
+ Typos and other minor fixes.

Pick-to: 6.0
Change-Id: I1ac8afebbdbea50549a334c07b8b3117a823f0fe
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

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