Fail fast when run under cross-platform PowerShell on Linux/macOS (rather than
fetching the Windows .exe), treating undefined $IsWindows on Windows
PowerShell 5.1 as Windows. Make the printed install example create the target
directory before moving the executable into it.
Add install-mac.sh (downloads the Apple Silicon binary, verifies SHA-256,
clears the macOS quarantine attr, prints the move-to-PATH command) and
install-windows.ps1 (downloads the .exe, verifies SHA-256, prints the
move-to-PATH command). Both write to a predictable filename (buckit /
buckit.exe) and use the same atomic temp-then-move + checksum-gate flow as
install-linux.sh.
Publish both to gh-pages in the release workflow, and point install-linux.sh's
non-Linux error at the new scripts.
abuckit