CI: adapt for new macOS runner

- enforce x86-64 architecture
- use “sysctl -n hw.ncpu” instead of “nproc” (which has not been installed in
  the new runner)

(cherry picked from commit ba78d0c8)

.gitlab-ci.yml

@@ -54,8 +54,8 @@ build_macos_x86_64:
     - macOS
   script:
     - autoreconf -si
-    - './configure CC=gcc-12 CXX=g++-12 LEX=/usr/local/opt/flex/bin/flex YACC=/usr/local/opt/bison/bin/bison'
-    - make -j$(nproc)
+    - 'arch -x86_64 ./configure CC=gcc-12 CXX=g++-12 LEX=/usr/local/opt/flex/bin/flex YACC=/usr/local/opt/bison/bin/bison'
+    - arch -x86_64 make -j$(sysctl -n hw.ncpu)
   artifacts:
     paths:
       - src/dynare-preprocessor