Appveyor cronbuild upload failing due to Github release conflict

Xref original feature #1967

The latest cronbuild cronbuild-7.0.17823 failed to upload to Github from Appveyor:

https://ci.appveyor.com/project/DynamoRIO/dynamorio/builds/19682356
```
...
'build\build_*\DynamoRIO*.zip' path
10219Uploading artifacts...
10220[1/1] build\build_release-external-64\DynamoRIO-Windows-7.0.17823-0.zip (167,268,417 bytes)...100%
10221Deploying using GitHub provider
10222Creating "cronbuild-7.0.17823" release for repository "DynamoRIO/dynamorio" tag "cronbuild-7.0.17823" commit "ef20472cf4c1b9fd8a62e714e
42b6e22b2450077"...OK
10223Uploading "DynamoRIO-Windows-7.0.17823-0.zip" to release assets...OK
10224Updating "cronbuild-7.0.17823" release for repository "DynamoRIO/dynamorio" tag "cronbuild-7.0.17823" commit "ef20472cf4c1b9fd8a62e714e
42b6e22b2450077"...Error updating GitHub release: Validation Failed
```

I did hit this while first setting up Appveyor support for uploading to the same Github release that Travis had already set up. My notes from there say:

> Xref https://github.com/appveyor/ci/issues/367 but not very satisfying answer there.
>
> Using the same Release title (used tag b/c that's default for Appveyor and then I don't have to try to duplicate the tag computation logic there) => it works!

I have no idea why it suddenly failed here after working for months. Filing now and we'll see if it fails next week.