ci/b2c: add support for the new format of CI_RUNNER_TAGS

ci/b2c: add support for the new format of CI_RUNNER_TAGS

Seems like Gitlab did not follow their own documentation:

```
CI_RUNNER_TAGS  8.10  0.5  A comma-separated list of the runner tags.
```

But it would seem like a gitlab update brought json-style lists which
broke:

```
CI_RUNNER_TAGS = '["keywords-gateway", "CI-gateway"]'
```

This commit adds support for both style.

Signed-off-by: Martin Roukala (né Peres) <martin.roukala@mupuf.org>
Reviewed-by: John Brooks <john@fastquake.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18923>