release: use old bullseye container for release build

[martinetd]

Building with trixie requires a recent glibc/stdc++ environment
(GLIBC_2.38 / GLIBCXX_3.4.32), which might not be available on the
target environment.

Conversely, as far as libc/stdlib are concerned we are guarnateed to be
able to run an old binary on a newer system, so building on a system as
old as possible should address this particular issue.

Unfortunately libflutter*so also link against system libraries
(GL, X, wayland, fontconfig and many others), so if there is any so bump
or imcompatible ABI then this is still far from perfect:
future improvement should rebuild flutter-embedded-linux on demand for
the required target, from flutter-elinux's CMake configuration

The elinux embedded itself is not hard to build but libflutter_engine.so
will be more work, so settle with "back to the old state" level for now.

---

Bullseye build broke on a single point, so there's a prerequisite patch, but it's trivial enough and should not be a problem.

I've tested running this on bullseye and on trixie so don't expect any problem with anything in between, this will be enough for the next immediate release.

I was also more annoyed than I thought I'd be at the bigger flutter engine .so release size since 3.32 (for example the arm64 debug libflutter_engine.so went from 83MB to 385MB!!!), so I'd like to have a look at that before the next release....
Hopefully by next week.

Cc @kabdelhalem -- thanks again, sorry to override your PR

[kabdelhalem]

Thanks @martinetd! No worries about overriding the PR - appreciate the quick turnaround on this, looking forward to the next release!

[martinetd]

Thanks!

I figured out why the release got much bigger too and opened #11 for that -- looks like a flutter build system bug...
Will let this sit over the weekend and merge/tag a release early next week