ENH: cmake: Ensure "slicerExtensionWizard --describe /path/to/dcmqi" works
ENH: cmake: Ensure "slicerExtensionWizard --describe /path/to/dcmqi" works

Since the wizard directly extract the metadata from the CMakeLists.txt,
this commit explicitly sets "EXTENSION_BUILD_SUBDIRECTORY"

[ci skip]

QIICR/dcmqi,QIICR/dcmqi,QIICR/dcmqi,QIICR/dcmqi,QIICR/dcmqi