feat(fish): add Linux x86_64 and aarch64 binary support

[detox-24]

1. Explicitly tagged 'x64' to 'amd64' in test.js to ensure arch names consistency across different helper functions.
2. Mapped newly released fish binaries for Linux with appropriate architectures in releases.js.
3. Added _linux_post_install() function in install.sh to display post-installation instructions in Linux.
4. Updated pkg_install() with linux-specific installation procedures

[detox-24]

@coolaj86 You think the fish docs need an update? The Linux instructions are already included, so I think it's pretty sufficient as it is.