fix(env): scope aws-vault file backend to non-macOS

macOS defaults to the keychain backend, which integrates with Touch ID;
unconditionally forcing the file backend hid credentials already stored
in ~/Library/Keychains/aws-vault.keychain-db. Branch on uname so Linux
hosts (e.g. SSH targets) keep the file backend while macOS reverts to
keychain.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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