Fix macOS installer and paths (#16251)

[Cid-oe]

Resolves Scottcjn/rustchain-bounties#16251

Fixes #7975

Changes:

• Fixed incorrect paths for macOS miner versions (selects v2.4.py for Intel x86_64 and v2.5.py for Apple Silicon arm64).
• Added strict OS detection to fail loudly and prevent half-installation on unsupported platforms (like FreeBSD or Windows).
• Cleaned up OS checks and package manager assumptions (brew is used for macOS python installation).
• Addressed all POSIX compliance issues. Shellcheck now passes with absolutely zero errors.
• Checked there is no usage of GNU-only commands like sed -i or grep -P.

Testing Evidence:

• Tested shellcheck on both install.sh and scripts/install.sh. Passed cleanly.
• Verified platform detection conditions cover macOS variations appropriately.

[github-actions]

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