Change logic for calling the diagnostic script to avoid problems with scripts where the executable bit is accidentally set

[bouweandela]

By always using the extension as a guideline on what interpreter to use before checking the executable bit, we avoid problems for users who set the wrong executable bit or are unable to control this (e.g. NTFS filesystems on Windows)

Tasks

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Closes #828

[stefsmeets]

Looks good, works with my use-case!