Simply adding `--sign` to the `makepkg` flags doesn't work because it would
require setting up GPG within the chroot environment (of `makechrootpkg`).
When debugging it is anyways annoying that `makepkg` sends the `gpg` output
to `/dev/null`. This way the logs are preserved.
* Can not use a normal mutex because we don't want to tie the resources to
a specific thread (and instead e.g. to a build action which might not be
executed by a single thread)
* A semaphore would do that but libstdc++ only supports it as of GCC 11 and
besides it wouldn't distinguish between shared and exclusive locking
Martchus