Describe the bug

If your machine has an environmental variable "projects" defined, restore.cmd fails.

I have the projects environmental variable defined to instruct Windows Defender to ignore my build output folder. (Otherwise
Windows Defender will delete App_global.asax.dll in my .net framework projects).

When attempting to build a fresh install of the aspnetcore project, I run restore.cmd. This installs a significant amount of software, but then fails because it says a project file completely outside of the aspnetcore repo is missing.

To Reproduce

git clone https://github.com/dotnet/aspnetcore.git
{ git clone output }
git submodule update --init --recursive
{ git update output }
restore.cmd
{ a bunch of output }
...
dotnet-install: .NET Core Runtime version 7.0.0-alpha.1.21419.1 is already installed.
dotnet-install: Adding to current process PATH: "C:\temp\20210826\aspnetcore.dotnet\x86". Note: This change will not be visible if PowerShell was run as a child process.
dotnet-install: Note that the intended use of this script is for Continuous Integration (CI) scenarios, where:
dotnet-install: - The SDK needs to be installed without user interaction and without admin rights.
dotnet-install: - The SDK installation doesn't need to persist across multiple CI runs.
dotnet-install: To set up a development environment or to run apps, use installers rather than this script. Visit https://dotnet.microsoft.com/download to get the installer.

**dotnet-install: ASP.NET Core Runtime version 3.1.14 is already installed.
C:\Users\GregH.nuget\packages\microsoft.dotnet.arcade.sdk\6.0.0-beta.21420.4\tools\Build.proj(221,5): error MSB3202: T
he project file "c:\temp\rocket_build" was not found.

Build FAILED.

C:\Users\GregH.nuget\packages\microsoft.dotnet.arcade.sdk\6.0.0-beta.21420.4\tools\Build.proj(221,5): error MSB3202: T
he project file "c:\temp\rocket_build" was not found.
0 Warning(s)
1 Error(s)

Time Elapsed 00:00:08.51
Build failed with exit code 1. Check errors above.**

set | grep -i build
Projects=c:\temp\rocket_build\

set projects=

restore.cmd
{ a bunch of output }
...
Build succeeded.

Further technical details

• ASP.NET Core version
  c:\temp\20210826\aspnetcore>git show
  commit 9611dc1 (HEAD -> main, origin/main, origin/HEAD)

• Include the output of dotnet --info

c:\temp\20210826\aspnetcore>dotnet --info

Host (useful for support):
Version: 5.0.9
Commit: 208e377a53

.NET SDKs installed:
3.1.412 [C:\Program Files\dotnet\sdk]
5.0.206 [C:\Program Files\dotnet\sdk]
5.0.301 [C:\Program Files\dotnet\sdk]
5.0.303 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.All 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.29 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.29 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.18 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.29 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.18 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.16 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.18 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.7 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.9 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET runtimes or SDKs:
https://aka.ms/dotnet-download

• The IDE (VS / VS Code/ VS4Mac) you're running on, and its version
  Haven't got to an IDE yet.
  VS2019 won't build it, but I'm assuming restore.cmd needs to complete without trying to compile some other non-existing project first.