mise.run script should check for an existing mise in the PATH before installing

[autarch]

It'd be nice if the script would check for an existing mise and not re-install if the binary matches the requested version. This could be an opt-in thing via an env var if you don't want this to be the default.

[JamBalaya56562]

This is now supported 🎉 — merged in #10882 (shipping in the next release).

The mise.run standalone installer gained an opt-in env var, MISE_INSTALL_SKIP_IF_EXISTS. When set to 1 or true, the script checks the mise binary at the install path and skips the download/install entirely if its version already matches the requested version:

curl https://mise.run | MISE_INSTALL_SKIP_IF_EXISTS=1 sh

It stays opt-in so the default behavior is unchanged. A note on the "in the PATH" part of the title: the check is scoped to the resolved install path (MISE_INSTALL_PATH, default ~/.local/bin/mise) rather than the wider PATH. That's deliberate — if the skip fired on a matching mise found elsewhere on PATH, the script could exit successfully without ever creating install_path, which would break the mise activate line the installer prints (and would ignore an explicitly-set MISE_INSTALL_PATH). For the common CI/Docker case the install always targets the same path, so this still avoids the redundant re-download.

Docs: https://mise.jdx.dev/installing-mise.html

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