fix(installer): select ARMv6 binary on armv6l

[gauravch-code]

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Description

Fixes #156.

Detect Linux armv6l separately so the installer downloads
pangolin-cli_linux_arm32v6 rather than the ARMv7 binary.
Other platform mappings remain unchanged.

Add isolated platform-detection tests and run them in CI.
The tests do not download or install Pangolin.

How to test?

• Run sh scripts/test-get-cli.sh: all 12 checks pass.
• Run sh -n get-cli.sh scripts/test-get-cli.sh: passes.
• The ARMv6 regression fails against the original installer.
• Confirmed the ARMv6 asset exists in release 0.18.1.

Not tested on physical Raspberry Pi hardware.