CI: add MTK_PANTHER_OPEN job to build SDK as public user, remove `allow_failure` in X86 job
We'd like to regularly test that the public SDK builds correctly, without having access to any private repositories.
MTK_PANTHER
board doesn't require any private components, hence it's a good candidate for this kind of testing.
Additionally, remove the allow_failure: true
parameter for the X86
job, since it's now considered stable.