On GNU/Linux, pick x13 binary from the path if it is there

As a consequence, no need to download the binary on the CI runners, since it is
already there.
2 jobs for master in 1 minute and 6 seconds
Status Job ID Name Coverage
  Test
passed testsuite_matlab #1781

00:00:58

failed testsuite_octave #1782
allowed to fail

00:01:06

 
Name Stage Failure
failed
testsuite_octave Test
warning: concatenation of different character string types may have unintended consequences
warning: called from
num2str at line 132 column 11
display_report at line 42 column 9
runalltests at line 67 column 1
error: horizontal dimensions mismatch (262x165 vs 261x10)
Makefile:7: recipe for target 'check-octave' failed
make: *** [check-octave] Error 1
ERROR: Job failed: exit status 1