Add (mostly static) Linux build

[fwcd]

This adds an experimental x86_64 Linux build, which should lay the foundation for #36 too. Cross-compilation will likely become a bit more involved on Linux compared to macOS since the system doesn't ship all of the target libraries natively. It currently uses a patched integration branch of Mixxx to fix some linking issues that arise due to most dependencies being linked statically (which is unlike the standard Debian build environment that Mixxx usually provides).

Upstreaming progress:

• Add release-mode x64-linux overlay triplet mixxxdj/vcpkg#95
• MixxxApplication: Support linking Qt statically on Linux mixxxdj/mixxx#12284
• FindPortAudio: Link ALSA in static builds on Linux mixxxdj/mixxx#12291
• FindPortMidi: Link ALSA in static builds on Linux mixxxdj/mixxx#12292
• FindLibudev: Link hidapi and libusb with libudev in static builds on Linux mixxxdj/mixxx#12294
• FindSleef: Use OpenMP in static builds mixxxdj/mixxx#12295
• FindVorbis: Link ogg in static builds mixxxdj/mixxx#12297

To do:

• Experimentally update CI and scripts for building on/for Linux
• Figure out why we still get errors while linking Mixxx (see the linked PRs above)

Since statically linking Mixxx on Linux does not seem to be a well-supported scenario by upstream, an alternative would be to build with the standard Debian environment and then bundling the shared libraries (e.g. copying most of the libraries listed by ldd into an AppImage or similar).

[fwcd]

We'll disable set battery indicator by setting -DBATTERY=OFF to avoid the GLib dependency, which is tricky to get right in a static linking scenario:

/usr/bin/ld: /home/runner/work/m1xxx/m1xxx/vcpkg/installed/x64-linux/lib/libgobject-2.0.a(gboxed.c.o): in function `g_bookmark_file_get_type_once':
gboxed.c:(.text+0x105a): undefined reference to `g_bookmark_file_copy'
collect2: error: ld returned 1 exit status

We would probably have to tell CMake to link glib with gobject similar to the other PRs linked above, the FindGLIB module is structured a bit differently, however, so it may not be trivial. Since this feature is rather minor, it might not be worth the hassle for this non-standard build.

[fwcd]

It works: