An earlier version of #16546 used both --with-boost-process and --enable-external-signer, which was simplified to only use the latter. However I forgot to update CI, so the external signer tests were not run.
Compilers used change as follows:
Linux native GCC 7.5 -> GCC 8.4
Linux cross GCC 8.4 -> GCC 8.4
Windows mingw-w64 7.3 -> mingw-w64 9.3
macOS Clang 8.0.0 -> Clang 8.0.0
The macOS and Win cross builds in the CI are updated to use Focal, and
per the op, running the security tests is disabled in the Windows
build.
xorriso and its mkisofs/genisoimage emulation alter-ego xorrisofs are
more maintained, and has the right toggles for us to achieve output
determinism without using blunt tools like faketime.
In this commit, we use xorrisofs from the build environment rather than
building it ourselves using depends. This is not necessary and can be
changed in the future.
From https://wiki.debian.org/genisoimage?action=recall&rev=11 :
> The classical command line interface for production of ISO 9660
> filesystem images is the option set established by program mkisofs.
> For reasons of licensing and other problems with its author, Debian
> ships a fork of mkisofs, called genisoimage, which was split off in
> 2006 and then developed independently.
>
> Meanwhile, genisoimage gets no new features and not even bug fixes. It
> is first choice only if its options -udf or -hfs are needed.
>
> Replacement in most uses cases, especially for bootable ISO 9660
> filesystems, archiving, and backup, is xorrisofs which starts the -as
> mkisofs emulation mode of program xorriso.
