CI: Test our Debian package for/with upcoming Debian Trixie

[hacklschorsch]

We want to be in the upcoming Debian Trixie, and Andrius, maintainer of the Tahoe-LAFS package for Debian, has been phenomenal making it happen.

Let's make sure stuff doesn't break on our side (or rather that we fix it quickly if it does) by integrating a CI runner for the package with our CI.

These are instructions I received from Andrius:

---

After that the instructions for Docker should be pretty simple:

1. Take Tahoe-LAFS source

2. Take debian/ directory from Debian packaging repository [1]

3. 'apt-get install' dependencies from debian/control

4. 'apt-get install quilt'

5. 'quilt push -a' (this might complain about inability to find patches,

but then 'ln -s debian/patches' should help)

4. Run 'make -f debian/rules binary'

If the build process reaches 'dh_testroot' (after that it will surely fail, but this is enough), this means the build and tests are successful.