oeqa/selftest/reproducibility: restart bitbake for each build

oeqa/selftest/reproducibility: restart bitbake for each build

Since [0], the reproducibility test tries to save logs of failed builds.
Using the memory persistent bitbake prevent this by concatenating logs in the
single persistent instance (See [YOCTO #15641]).
So, force a BitBake server restart for each build to ensure a new log
file is created.

This fixes this error (seen on AB [1]):
  2024-11-02 08:16:00,816 - oe-selftest - ERROR - reproducibleA build failed. Trying to compute built packages differences but the test will fail.
  2024-11-02 08:16:00,819 - oe-selftest - INFO -  ... ERROR
  2024-11-02 08:16:00,819 - oe-selftest - INFO - Traceback (most recent call last):
    File "/srv/pokybuild/yocto-worker/reproducible-meta-oe/build/meta/lib/oeqa/selftest/cases/reproducible.py", line 321, in test_reproducible_builds
      self.copy_file(variables["BB_CONSOLELOG"], os.path.join(save_dir, "bitbake-%s.log" % name))
    File "/srv/pokybuild/yocto-worker/reproducible-meta-oe/build/meta/lib/oeqa/selftest/cases/reproducible.py", line 216, in copy_file
      shutil.copyfile(source, dest)
    File "/usr/lib/python3.11/shutil.py", line 256, in copyfile
      with open(src, 'rb') as fsrc:
           ^^^^^^^^^^^^^^^
  FileNotFoundError: [Errno 2] No such file or directory: '/srv/pokybuild/yocto-worker/reproducible-meta-oe/build/build/build-st-meta-python/build-st/tmp/log/cooker/qemux86-64/20241102071141.log'

[0]: https://git.openembedded.org/openembedded-core/commit/?id=c78cc753843d4199443052e8deb0c9c3b7e4b580
[1]: https://valkyrie.yoctoproject.org/#/builders/87/builds/17/steps/40/logs/stdio

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>