Fix failing zephyr CI workflows, pinning v0.27.4

[bhess]

The Zephyr build has been failing for the past week during CI initialization of the container.

The failure seems to correlate with the "latest" update to the Zephyr container that occurred 6 days ago: https://github.com/zephyrproject-rtos/docker-image/pkgs/container/ci.

The PR fixes the failing zephyr worflow by pinning the Zephyr container to the latest release 0.27.4, which would bring CI to a better state. However, CI overall is still failing due to #2056.

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider will also need to be ready for review and merge by the time this is merged.)