adding RuntimeIdentifiers to the fsproj breaks the build

Description

I came across some really strange behaviour when using paket and dotnet core together with RuntimeIdentifiers.

Repro steps

Please provide the steps required to reproduce the problem

1. Create a new hello world fsproj

2. add a RuntimeIdentifiers section to the file:

    <RuntimeIdentifiers>ubuntu.16.04-arm;ubuntu.14.04-arm</RuntimeIdentifiers>

3. run paket install

4. run dotnet build

Expected behavior

build is successful.

Actual behavior

build fails with error message:

/Users/kdummann/source/rubecula/frontend/backend/backend.fsproj : error NU1100: Unable to resolve 'runtime.linux-arm.Microsoft.NETCore.App (>= 2.0.0)' for '.NETCoreApp,Version=v2.0/ubuntu.14.04-arm'.
/Users/kdummann/source/rubecula/frontend/backend/backend.fsproj : error NU1100: Unable to resolve 'runtime.linux-arm.Microsoft.NETCore.DotNetHostPolicy (>= 2.0.0)' for '.NETCoreApp,Version=v2.0/ubuntu.14.04-arm'.
/Users/kdummann/source/rubecula/frontend/backend/backend.fsproj : error NU1100: Unable to resolve 'runtime.linux-arm.Microsoft.NETCore.DotNetHostResolver (>= 2.0.0)' for '.NETCoreApp,Version=v2.0/ubuntu.14.04-arm'.
/Users/kdummann/source/rubecula/frontend/backend/backend.fsproj : error NU1100: Unable to resolve 'runtime.linux-arm.Microsoft.NETCore.DotNetAppHost (>= 2.0.0)' for '.NETCoreApp,Version=v2.0/ubuntu.14.04-arm'.
/Users/kdummann/source/rubecula/frontend/backend/backend.fsproj : error NU1100: Unable to resolve 'runtime.linux-arm.Microsoft.NETCore.App (>= 2.0.0)' for '.NETCoreApp,Version=v2.0/ubuntu.16.04-arm'.
/Users/kdummann/source/rubecula/frontend/backend/backend.fsproj : error NU1100: Unable to resolve 'runtime.linux-arm.Microsoft.NETCore.DotNetHostPolicy (>= 2.0.0)' for '.NETCoreApp,Version=v2.0/ubuntu.16.04-arm'.
/Users/kdummann/source/rubecula/frontend/backend/backend.fsproj : error NU1100: Unable to resolve 'runtime.linux-arm.Microsoft.NETCore.DotNetHostResolver (>= 2.0.0)' for '.NETCoreApp,Version=v2.0/ubuntu.16.04-arm'.
/Users/kdummann/source/rubecula/frontend/backend/backend.fsproj : error NU1100: Unable to resolve 'runtime.linux-arm.Microsoft.NETCore.DotNetAppHost (>= 2.0.0)' for '.NETCoreApp,Version=v2.0/ubuntu.16.04-arm'.

The behaviour is not arm specific it also happens for x64 and x86.

Known workarounds

do not use paket to do nuget handling. Simply removing the restore targets file from the fsproj file is enough. Run dotnet restore or dotnet build. Then add the paket restore targets again and everything works. This seems to fix it for the whole machine. The problem is more in CI where we run each build in a new container and as a workaround have to run dotnet restore in a dummy project to get the code building.

Start with the hello-world .fsproj and its Paket restore targets file, then reproduce the issue with paket install followed by dotnet build using the listed RuntimeIdentifiers. Compare this with the successful dotnet restore workaround. Done means the project builds through Paket without the NU1100 errors for the listed runtimes.