Installer aborts on an existing same-named `slack` binary; prefer install + warning over hard abort

[curusarn]

What happens

install.sh refuses to install if any slack already exists on PATH. On a machine where the slack-ruby-client gem provides a slack executable (an rbenv shim at ~/.rbenv/shims/slack), the installer aborts:

🛑 Error: Your existing `slack` command is different from this Slack CLI!
🔖 Try using an alias when installing to avoid name conflicts:
   curl -fsSL https://downloads.slack-edge.com/slack-cli/install.sh | bash -s your-preferred-alias

Repro

1. Have an unrelated slack on PATH. Concrete case: gem install slack-ruby-client (ships a slack executable; under rbenv it becomes ~/.rbenv/shims/slack).
2. curl -fsSL https://downloads.slack-edge.com/slack-cli/install.sh | bash
3. Installer aborts instead of installing.

Why this is unhelpful

Aborting on a same-named binary is non-standard. It forced installing under an alias (| bash -s slack-cli), then manually symlinking it back to slack and reordering PATH, because rbenv shims sit ahead of ~/.local/bin, so even after installing, the CLI stayed shadowed by the gem's slack. Every other CLI installer I use just installs the binary and leaves PATH precedence to me.

Suggestion

Install regardless: place the binary + symlink as usual, and print a warning about the name collision (keep the alias tip there), instead of hard-aborting. Let the user manage their own PATH.

Env

macOS arm64, Slack CLI v4.7.0, zsh + rbenv, slack-ruby-client 2.6.0