Enforce metakernel version in the examples
The latest JUICE CReMA 5.0
(v310 and v320) generate an error in multicore downloads during the CI due to a duplication of the file juice_mga_crema_5_0_default_f20160326_v01.bc
in the metakernel. This issue was reported to ESA Spice Service and in https://juigitlab.esac.esa.int/datalab/moon-coverage/-/issues/51.
This MR enforce an older version v300 of the 5.0
metakernel to avoid this issue.