"Build and Pack Nuget" Doing No Work in PR Pipeline (Effected by Enabling LayoutHeaders)

[NickGerleman]

We seem to at least sometimes completely no-op on "Build and Pack Nuget" for both Desktop and UWP. E.g. this build

Oddly enough, enabling LayoutHeaders for UWP x86 Release, as in #3888, causes us to actually run, for both UWP and desktop. Doing this, we see issues trying to pack ReactUWP. This is despite the build step to patch the NuSpec to exclude ReactUWP. It seems like this patching step may be broken, but we hadn't noticed because we weren't running it. Build Link

[NickGerleman]

Removing LayoutHeaders from the build matrix moved us back to the first state where we no-op, even for Desktop.