.gitlab-ci.yml: Silence pywriter harder

.gitlab-ci.yml: Silence pywriter harder

"make -j$(nproc) pywriter" and "pywriter" fill the log with over 20000
lines like this:

> /builds/neuschaefer/soc/openpower-isa/src/openpower/decoder/power_decoder.py:420: UnusedElaboratable: <openpower.decoder.power_decoder.PowerDecoder object at 0x7f4bf8ae8e48> created but never used
>   row_subset=self.row_subsetfn)
> UnusedElaboratable: Enable tracemalloc to get the object allocation traceback

Ideally, the relevant code should be fixed to avoid this warning, but for now,
I'm silencing it, so I can fit more relevant output into GitLab-CI's 4 MiB limit.