@x13/run.m: compatibility fix for Octave

Previously, .mdl files would unnecessarily be passed through importdata(),
which crashes on Octave.

The nested “if”s (testing for the output file extension) have been reorganized,
so that importdata() is not called on .mdl files (and neither on .acm and .rcm,
where is not needed either).
parent fc358e91
Pipeline #1872 passed with stage
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