Use artifacts from Coherence-Signed for installer generation and fallback to myget if it's not available

[JunTaoLuo]

Follow up to #154 (comment). Currently this depends on Coherence-Signed, we want to be able to run this locally with unsigned bits when building from source.

[JunTaoLuo]

The debian installer generation is currently standalone and does not depend on Coherence-Signed as it downloads the runtime store archives from myget instead of the artifacts from Coherence-Signed.

This however leads to a problem in our builds where the latest archives are not being used to build the installers due to the indexing delays of myget. For example in the first 25794 build of the installers, the 25778 archives were used: http://aspnetci/viewLog.html?buildId=244765&buildTypeId=Coherence_RuntimeStoreInstallers&tab=buildLog&state=&expand=all#_focus=327.

Instead, the work that needs to be done here is to add a mechanism to retrieve the archives from Coherence-Signed but fallback to myget if it's not available so the installer builds remain standalone.