perf tests: add fallback feed to dotnet restore (#3160)
perf tests: add fallback feed to dotnet restore (#3160)

The perf tests use `dotnet restore` to download test dependencies. The
perf tests' project.json may sometimes refer to a version of the
framework that is not yet published, which can cause the restore command
to fail. This change addresses the issue by adding a fallback nuget feed
that contains not-yet-published version of the framework.
mlorbetske/cli,naamunds/cli,svick/cli,borgdylan/dotnet-cli,weshaggard/cli,EdwardBlair/cli,naamunds/cli,livarcocc/cli-1,mylibero/cli,MichaelSimons/cli,jonsequitur/cli,mylibero/cli,FubarDevelopment/cli,ravimeda/cli,harshjain2/cli,JohnChen0/cli,weshaggard/cli,JohnChen0/cli,dasMulli/cli,weshaggard/cli,nguerrera/cli,ravimeda/cli,mlorbetske/cli,svick/cli,naamunds/cli,Faizan2304/cli,dasMulli/cli,weshaggard/cli,AbhitejJohn/cli,borgdylan/dotnet-cli,mlorbetske/cli,FubarDevelopment/cli,harshjain2/cli,MichaelSimons/cli,AbhitejJohn/cli,johnbeisner/cli,blackdwarf/cli,FubarDevelopment/cli,FubarDevelopment/cli,jonsequitur/cli,nguerrera/cli,nguerrera/cli,EdwardBlair/cli,JohnChen0/cli,mlorbetske/cli,johnbeisner/cli,mylibero/cli,naamunds/cli,JohnChen0/cli,blackdwarf/cli,borgdylan/dotnet-cli,borgdylan/dotnet-cli,blackdwarf/cli,weshaggard/cli,jonsequitur/cli,AbhitejJohn/cli,livarcocc/cli-1,MichaelSimons/cli,dasMulli/cli,ravimeda/cli,MichaelSimons/cli,nguerrera/cli,svick/cli,jonsequitur/cli,Faizan2304/cli,EdwardBlair/cli,MichaelSimons/cli,naamunds/cli,AbhitejJohn/cli,mylibero/cli,harshjain2/cli,johnbeisner/cli,borgdylan/dotnet-cli,blackdwarf/cli,livarcocc/cli-1,Faizan2304/cli,mylibero/cli