Set DOTNET_CLI_HOME to a repo-local path

[dsplaisted]

• This issue is blocking
• This issue is causing unreasonable pain

By default, the CLI will use a user-specific folder for a few things, including first run sentinels and global tool information. The folder can be overridden with the DOTNET_CLI_HOME environment variable. Arcade should set this variable to a repo-local path, to avoid different builds (ie on CI machines or build servers) sharing this folder. While we should generally keep compatibility, that may not always be true between interim preview builds.

This has broken builds at least twice:

• SDK with net5.0 change fails to restore dotnet repo tools sdk#10938 (comment)
• Upgrade to more recent .NET SDK aspnetcore#21017 (comment)

[ViktorHofer]

@janvorli that's the issue that is tracking the right fix for the issue that you were seeing locally. cc @jaredpar

Running this test build where I set the variable to the build workspace directory: https://dev.azure.com/dnceng/internal/_build/results?buildId=724637&view=results

Ahh, forgot that arcade doesn't have a linux build leg. Will test this in arcade-validation instead.

PR to set this during CI: #5775

My fix doesn't work for docker containers that are started during the build, as oposed as having the entire leg running inside a container. Reopening.

https://dev.azure.com/dnceng/internal/_build/results?buildId=735034&view=logs&j=b72e85ab-3386-5aa9-6405-3837662d9688&t=f60252b2-6cdd-5047-3d04-b33edb9d8faa

Made a better fix, which sets the variable to the root of the repo during CI in #5805