Publish checksum.txt file with each release

It would be nice if there was a checksum.txt file with the contents of the sha256 checksums of each released binary.

e.g. for the 1.11.0 release
```
export BAZELISK_VERSION=1.11.0
for i in bazelisk-darwin bazelisk-darwin-amd64 bazelisk-darwin-arm64 \
bazelisk-linux-amd64 bazelisk-linux-arm64 bazelisk-windows-amd64.exe; do
curl -LO https://github.com/bazelbuild/bazelisk/releases/download/v${BAZELISK_VERSION}/${i}
done
sha256sum bazelisk-* >checksums.txt
```

Checksums.txt contents
```
afb0857e4497810dd9695af2b1491c4184debe45ec035a2bb8a4aeb1542166c4 bazelisk-darwin
c725fd574ea723ab25187d63ca31a5c9176d40433af92cd2449d718ee97e76a2 bazelisk-darwin-amd64
1e18c98312d1a03525f704214304be2445478392c8687888d5d37e6a680f31e6 bazelisk-darwin-arm64
231ec5ca8115e94c75a1f4fbada1a062b48822ca04f21f26e4cb1cd8973cd458 bazelisk-linux-amd64
f9119deb1eeb6d730ee8b2e1a14d09cb45638f0447df23144229c5b3b3bc2408 bazelisk-linux-arm64
0eca0630e072434c63614b009bf5b63a7dff489a8676b560ce63e0d91ab731cd bazelisk-windows-amd64.exe
```

The issue does not name a file or release entry point. Start by tracing how Bazelisk binaries are built and published, then add a checksum.txt artifact containing SHA-256 checksums for each released binary. Done means every release includes the checksum file and its entries verify the corresponding downloads.

Written by the indexing model from the issue text.