CI: use system-wide Octave binary

.gitlab-ci.yml

 variables:
   MATLAB_VERSION: R2022b
   OLD_MATLAB_VERSION: R2014a
-  OCTAVE_VERSION: 7.3.0
 
 before_script:
   - git clone https://git.dynare.org/Dynare/dseries.git
@@ -25,7 +24,7 @@ test_old_matlab:
 test_octave:
   stage: test
   script:
-    - make check-octave OCTAVE=/usr/local/octave/$OCTAVE_VERSION/bin/octave
+    - make check-octave
   artifacts:
     paths:
       - test/my/report/dir/report.pdf