Fix metakernels URLs

When a metakernel does not have an explicit remote, the Metakernel object does not find the missing kernels correctly and does not raise the expected MissingKernelsRemote error.

TourConfig(
    mk=ENVISION_CREMAS['envision_study_south_voi_v010.tm'],
    spacecraft='EnVision',
    target='Venus',
    download_kernels=True,
)

spiceNOSUCHFILE: 
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Toolkit version: CSPICE66

SPICE(NOSUCHFILE) --

The seventh file 'spk/EnVision_ESC_T2_2032_SouthVOI.bsp' specified by KERNELS_TO_LOAD
in the file /tmp/envision_study_south_voi_v010-xxxxxxxx.tm could not be located.

furnsh_c --> FURNSH --> ZZLDKER

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