KAuth buildability: new CI architecture

[Harald Sitter]

Not particularly related to the issue at hand (which is probably
polkitqt having meh cmake files), but relocating stuff in general is
suuuuper unreliable and I would absolutely advise against it as it can
easily screw up test results and builds alike, often in unobvious ways
(all it takes is a bit of libexec use). Instead, as a general
principle, I would suggest that you get stuff mounted in suitably
stable/consistent/generic paths inside the build containers.
Ultimately what things look like natively on the host file system
shouldn't factor into what they look like in the build environment.

For example, in neon we simply mount everything into /workspace of our
containers.

You could have

/workspace
|_ src/      [on host: /home/jenkins/workspace/polkit-qt-1 kf5-qt5 XenialQt5.7/
|_ install/  [on host: /home/jenkins/workspace/polkit-qt-1 kf5-qt5
XenialQt5.7/install-prefix/]

Notably advantage is that relocation issue get entirely eliminated as
the install prefix is always the same, and as an added bonus paths
become much shorter and easier to read in the build logs.

It also detaches the build tooling from the host tooling. e.g. the
build code at this point no longer needs to care about which path
Jenkins was installed to, or where it was configured to put
workspaces, or what the jenkins job name is, or if jenkins is even
still used.

Food for thought :)

HS