endpoints/cli: Make installer OS param accept `uname -s` output…

[tsibley]

…by making it case-insensitive (so "Linux" matches "linux") and accepting "Darwin" as alias for "mac".

Accepting uname -s make it easier to give cross-platform instructions for Linux/macOS. This is intended for ad-hoc installations for internal usage by developers; it's not a suggestion to change our general installation docs.

Checklist

• [x] Checks pass
• [ ] Check if changes affect the resource index JSON revision