Move NO_SI to the standalone server only, which is where I actually need it

src/calibre/srv/standalone.py

@@ -298,7 +298,7 @@ def main(args=sys.argv):
             raise SystemExit(_('You must specify at least one calibre library'))
         libraries=[prefs['library_path']]
 
-    if not singleinstance('db'):
+    if b'CALIBRE_NO_SI_DANGER_DANGER' not in os.environ and not singleinstance('db'):
         ext = '.exe' if iswindows else ''
         raise SystemExit(_(
             'Another calibre program such as another instance of {} or the main'

src/calibre/utils/lock.py

@@ -192,8 +192,6 @@ else:
 
 def singleinstance(name):
     ' Ensure that only a single process holding exists with the specified mutex key '
-    if b'CALIBRE_NO_SI_DANGER_DANGER' in os.environ:
-        return True
     release_mutex = create_single_instance_mutex(name)
     if release_mutex is None:
         return False