Record devpi index URL in requirements.txt files for reproducibility

src/xfel_calibrate/calibrate.py

@@ -715,6 +715,8 @@ def run(argv=None):
     # Record installed Python packages for reproducing the environment
     if not args['skip_env_freeze']:
         with (cal_work_dir / 'requirements.txt').open('wb') as f:
+            print('--index-url https://devpi.exfldadev01.desy.de/euxfel/internal',
+                  file=f, flush=True)
             check_call([python_exe, '-m', 'pip', 'freeze'], stdout=f)
 
     folder = get_par_attr(parms, 'in_folder', 'value', '')