On environments where registry access is disabled, the first run experience
fails because it could not add the tools path to the user's environment.
This fix properly handles the security exception by printing a warning and
continuing. Users will have to manually add the PATH environment variable to
their environments to prevent `dotnet tool install` from printing PATH
instructions.
A new file sentinel is added to track whether or not the PATH has been
modified. The first run experience also now correctly skips modifying the PATH
if `DOTNET_SKIP_FIRST_TIME_EXPERIENCE` is set.
Fixes#8874.
