fix: correct cog download URL for Linux arm64 (aarch64) (#3066)
The documented install commands and install.sh built the release asset URL
from `uname -m`, which reports `aarch64` on Linux arm64. The release assets
are named `arm64`, so the URL resolved to a non-existent `cog_Linux_aarch64`
asset and returned a 404.
Normalize `aarch64` -> `arm64` (and `amd64` -> `x86_64` in install.sh) so
the URL matches the published asset names.
Fixes #1309
Co-authored-by: Mark Phelps <209477+markphelps@users.noreply.github.com>
