Add selfinstall --skip-env to avoid shell/profile mutation

[Copilot]

dnvm selfinstall always updated the user environment, which made non-interactive bootstrap flows noisy and undesirable in CI. This change adds an explicit opt-out so callers can install the dnvm binary without modifying shell profiles or persistent user environment variables.

• CLI surface

  • Adds --skip-env to dnvm selfinstall.
  • Exposes the flag in command help alongside --skip-tracking.

• Self-install behavior

  • Threads the new option through SelfInstallCommand.
  • Suppresses the "update user environment" prompt when --skip-env is set.
  • Skips the UpdateEnv(...) path entirely, avoiding:
    • Unix shell profile edits and $DNVM_HOME/env generation
    • Windows persistent user PATH / DOTNET_ROOT updates

• Usage

  • Supports side-effect-free bootstrap when combined with existing tracking opt-outs:

curl --proto '=https' -sSf https://dnvm.net/install.sh | sh -s -- -y --skip-tracking --skip-env

• Coverage
  • Updates selfinstall help expectations.
  • Adds focused coverage for the Unix --skip-env path to ensure profile/env files are left untouched.