fix(aztec-up): fall back to no timeout when /usr/bin/timeout absent (macOS)

[nchamo]

Problem

The aztec-up installer wraps foundryup in a timeout shell function. The previous implementation only worked on Linux: it gated the call on CI=true and invoked /usr/bin/timeout directly. /usr/bin/timeout ships with GNU coreutils and is absent on stock macOS, so on a macOS GitHub Actions runner (CI=true is auto-set) the installer aborted in install_foundry with:

bash: line 59: /usr/bin/timeout: No such file or directory

Fix

Rework the timeout function so it picks a working backend at runtime. The CI gate is preserved — non-CI runs continue to execute unguarded, matching prior behavior:

• Prefer /usr/bin/timeout when present (Linux via coreutils).
• Fall back to perl -e 'alarm ...; exec ...' when /usr/bin/timeout is missing. Perl ships preinstalled on macOS and GitHub's Ubuntu runners, so the guard remains effective there.
• If neither backend is available, run the command unguarded rather than fail the install — the timeout exists to bound a `foundryup` hang in CI, not to gate the install itself.

Verified the three branches in containers (coreutils path, perl path, neither): timeouts fire at the expected duration, exit codes propagate, and the unguarded branch passes through completion.