project tests: handle code internally fetching the meson command

Fixes crash when running the tests and yasm is installed, but nasm is
not. When printing the compilers at early startup in
run_project_tests.py, we try to detect the nasm compiler, which in this
case turns out to be yasm, and its exelist assumes that the meson
command is set.

Fixes #12662

run_project_tests.py

@@ -1631,6 +1631,7 @@ if __name__ == '__main__':
 
     clear_transitive_files()
     setup_symlinks()
+    mesonlib.set_meson_command(get_meson_script())
 
     print('Meson build system', meson_version, 'Project Tests')
     print('Using python', sys.version.split('\n')[0], f'({sys.executable!r})')