"Not cryptographic hash found" when using --prefetch

### General summary/comments (optional)

I was trying to benchmark how long it takes to build pandoc. In order to eliminate the time it takes to download packages, I used

```
stack install pandoc --dry-run --prefetch -v
```

However, that results in

```
...
2020-11-15 22:35:01.157377: [info] Not cryptographic hash found for Hackage package Glob-0.10.1, updating
2020-11-15 22:35:01.157535: [info] Selected mirror https://s3.amazonaws.com/hackage.fpcomplete.com/
2020-11-15 22:35:01.157665: [info] Downloading root
2020-11-15 22:35:01.180305: [error] Not cryptographic hash found for Hackage package HTTP-4000.3.15
```

### Steps to reproduce

Prior to this, I only ran

```
stack setup --resolver=lts-16.21
stack install pandoc --dry-run -v
```

Full log: https://ro-che.info/tmp/stack_5Z7WCKF4CXVCX23HMQKPLG5XPMGVYVBJWMT7ESWPUFLESR5R.log

### Stack version

```
$ stack --version
Version 2.5.1, Git revision d6ab861544918185236cf826cb2028abb266d6d5 x86_64 hpack-0.33.0
```

### Method of installation

* Official binary, downloaded from stackage.org or fpcomplete's package repository