cat tries to load non-existent file when comparing sums

Saw this in output

```
multirust: downloading checksums for rust installer from 'http://static-rust-lang-org.s3-website-us-west-1.amazonaws.com/dist/rust-nightly-x86_64-unknown-linux-gnu.tar.gz'
multirust: downloading 'http://static-rust-lang-org.s3-website-us-west-1.amazonaws.com/dist/rust-nightly-x86_64-unknown-linux-gnu.tar.gz.sha256' to '/tmp/tmp.zbssz1cYk0/multirust-home/tmp/tmp-18486-6'
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 111 100 111 0 0 2286 0 --:--:-- --:--:-- --:--:-- 2312
cat: /tmp/tmp.zbssz1cYk0/multirust-home/channel-sums/nightly.sha256: No such file or directory
multirust: downloading rust installer from 'http://static-rust-lang-org.s3-website-us-west-1.amazonaws.com/dist/rust-nightly-x86_64-unknown-linux-gnu.tar.gz'
```

Start by reproducing the installer flow that compares checksums and trace how channel-sums/nightly.sha256 is handled. Inspect the checksum-download and comparison entry point; done means the flow no longer invokes cat on a missing file and proceeds with the installer behavior expected by the issue.

Written by the indexing model from the issue text.