fix: correct download URL pattern in install script

[TimPietruskyRunPod]

Summary

• Fix download URL to match goreleaser's naming: runpodctl-{os}-{arch}.tar.gz (dashes, no version in filename)
• Use darwin-all universal binary for macOS instead of per-arch builds
• Add aarch64/arm64 support for Linux

What was broken

The install script constructed URLs like:

runpodctl_1.14.15_linux_amd64.tar.gz  (underscores, version in filename)

But goreleaser produces:

runpodctl-linux-amd64.tar.gz  (dashes, no version)

This caused a 404, leaving the old pre-installed binary on PATH.

Validated

• macOS darwin-all URL → 200, binary runs correctly (runpodctl 2.1.0-0e6667c)
• Linux linux-amd64 URL → 200
• Linux linux-arm64 URL → 200

Fixes #221
Fixes #150