Sdk: dotnet tool install fails when there are multiple .csproj files in directory

[logankp]

Steps to reproduce

• Have multiple .csproj files in directory
• Run "dotnet tool install _package_ --version _version_ -g"

Expected behavior

Tool installs correctly.

Actual behavior

Tool install fails with the following error: "Specify which project file to use because this '/sln' contains more than one project file."

Environment data

.NET Core SDK (reflecting any global.json):
Version: 2.1.302
Commit: 9048955601

Runtime Environment:
OS Name: debian
OS Version: 9
OS Platform: Linux
RID: debian.9-x64
Base Path: /usr/share/dotnet/sdk/2.1.302/

Host (useful for support):
Version: 2.1.2
Commit: 811c3ce6c0

.NET Core SDKs installed:
2.1.302 [/usr/share/dotnet/sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.2 [/usr/share/dotnet/shared/Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.2 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.2 [/usr/share/dotnet/shared/Microsoft.NETCore.App]

[fbouteruche]

It is the same issue if you want to run a global tool in a folder that contains multiple project files.