Set DOTNET_CLI_HOME to a repo-local path

- [ ] 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:

- https://github.com/dotnet/sdk/issues/10938#issuecomment-601997145
- https://github.com/dotnet/aspnetcore/pull/21017#issuecomment-623261256