install.sh script should be arm64 aware

**Describe the problem/challenge you have**

The `install.sh` script referred to in Carvel web site instructions only installs binaries for AMD64. This means it installs binaries that don't work on ARM64 Linux, and doesn't install native ARM64 binaries on MacOS running Apple silicon.

**Describe the solution you'd like**

Should detect architecture and install ARM64 binaries when appropriate.

**Anything else you would like to add:**

This will require that `kwt` finally be released with ARM64 binary.

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Start by reading install.sh and trace how it selects the binary to download. Check how ARM64 binaries are expected to be named for Linux and macOS, and confirm the kwt ARM64 release prerequisite. Done means ARM64 systems receive native binaries while existing AMD64 installation continues to work.

Written by the indexing model from the issue text.