Don't publish -pre builds to MyGet

[CharliePoole]

Note: I documented the various errors arising in automatically publishing packages and doing the github release here: #1074 This relates to one of them.

We currently publish labels dev and pre to MyGet. This is really not necessary, as we don't do anything with those builds and they generally don't contain anything that's not already in a dev build. We've continued to do it primarily out of inertia.

However, the version of GitReleaseManager we are using has bugs in how it handles PRs and the build fails when it tries to push the PR build to MyGet. The easiest solution is to stop pushing those builds, at least until the GRM bug is fixed.

[CharliePoole]

This issue has been resolved in version 3.13.2

The release is available on:
GitHub.
NuGet packages are also available NuGet.org and
Chocolatey Packages may be found at Chocolatey.org