[ci] add extra monthly builds for older linux/glibc versions

[LinqLover]

Closes #742

Also includes minor revisions for linux prepare scripts:

• extract sudo from prepare scripts because docker does not need it (and specifying env vars like DEBIAN_FRONTEND from the outside is not possible if the script uses sudo without -E)
• add missing dev package libltdl-dev (seems to be preinstalled in GitHub runner but not a stock build-essentials Ubuntu image)
• ensure add-apt-repository is installed before using it for linux32x68 builds

When merging, please consider doing a squash merge to keep the ancestry clean.

[LinqLover]

@marceltaeumel Does this look okay, can we merge it? :)

@dtlewis290 If you need it, you can already download a working VM for GLIBC 2.31 from here :-)

[dtlewis290]

Excellent. I can confirm that your build artifact runs successfully on the older Linux image that we are using on squeaksource.com. This means that it resolves the glibc version issue on that Linux version 5.10.0-14-amd64 platform. This is a an older but still supported Linux with about 12 months remaining on its LTS lifespan, so it is likely typical of Linux versions that many Squeak end users will be using. I have so far tested only "./squeak -version" to confirm that the VM runs without link errors, and I will plan to do a test for real on the squeaksource.com server later on (maybe this weekend) when I can watch it and roll bock the change if any issues are seen.