Release simulation job

[gilles-peskine-arm]

When we prepare a release, we don't just release the tip of development: we do a few more things, such as assemble_changelog.py, make generated_files, merging quarantined security fixes, etc. In principle, doing these things should be ok. In practice, subtle bugs in test scripts can cause the CI to fail after preparing the release branch, even though it passed before. For example, we discovered Mbed-TLS/mbedtls#8300 while preparing the 3.5.0 release.

The goal of this issue is to set up a regular (nightly? weekly?) job that does a subset of release preparation, then runs the CI. Send reports over email like the nightly job that runs on the development branches. Among the things we do to prepare a release:

• MUST commit the generated files.
• Out of scope: merging restricted security fixes.
• Out of scope: bumping the version number.
• Out of scope: API/ABI checks.

Prerequisites:

• Result analysis misses generated test cases mbedtls#8300
• Remove trailing whitespace in prepare_release.sh mbedtls#8304

A likely subtask would be to add more stuff to scripts/prepare_release.sh.