Let --report=minimal be not entirely quiet

[marcelm]

Originally posted by @eboyden in https://github.com/marcelm/cutadapt/issues/685#issuecomment-1484470430:

I knew we were building an index but didn't initially see the message because I was running --report minimal. I turned that off and now see Building index of 43280 adapters .... Some of those summary messages might be useful to retain even when running --report minimal.