GHA: Add GCC 16 to CI

[Flamefire]

No description provided.

[Flow86]

The GCC 16 job currently fails before package installation because the new helper script is invoked directly but is not executable in the checkout:

tools/ci/add-apt-keys.sh: Permission denied Process completed with exit code 126.

The minimal fix would be to invoke both helper scripts through bash in unit-tests.yml:

* ```
           tools/ci/add-apt-keys.sh "${{matrix.apt_key}}"
  ```


* ```
           bash tools/ci/add-apt-keys.sh "${{matrix.apt_key}}"
  ```


* ```
           tools/ci/add-apt-repositories.sh "${{matrix.apt_repo}}"
  ```


* ```
           bash tools/ci/add-apt-repositories.sh "${{matrix.apt_repo}}"
  ```

That avoids relying on executable file mode for the newly added scripts. After that CI should at least get past the current immediate failure and show whether the PPA/key setup itself works.

please stop those AI stuff. its not helpful if a simple git update-index --chmod=+x $script would have been enough

[Flamefire]

please stop those AI stuff. its not helpful if a simple git update-index --chmod=+x $script would have been enough

Yeah, done that. CUrrently launchpad seems to be down, so it doesn't work right now