PR builds should not be published to BDN nightly feed

[mawosoft]

I'm opening this again (previously #1922) because it still seems to happen, plus there are some other shenanigans going on.

• PR Fix optimization of action methods for coreclr #1935 published version 0.13.1.1707 on the AppVeyor feed.
• But what's worse, subsequent new commits to an older PR Add CI for ARM32 #1077 were created with the same version 0.13.1.1707 and overwrote the already published version on the AppVeyor feed. Thus, the version number of the nightly builds becomes even more unreliable. Of course, NuGet still recognizes the changed SHA, but still...