Fix missing ipsw dependency

[BoringApp]

Summary

This PR improves the handling of the missing ipsw dependency when running JB setup.

Reproduction

make JB=1 setup_machine

Current behavior

The setup fails with:

=== CFW install ===
make[1]: Entering directory '/Users/a/b/c/vphone-cli'
cd vm && SSH_PORT="29182" _VPHONE_PATH="$PATH" zsh "/Users/a/b/c/vphone-cli/scripts/cfw_install_jb.sh" .
[*] cfw_install_jb.sh — Installing CFW + JB extensions...

[*] cfw_install.sh — Installing CFW on vphone...
[-] 'ipsw' not found. Install: brew install blacktop/tap/ipsw
make[1]: *** [Makefile:332: cfw_install_jb] Error 1
make[1]: Leaving directory '/Users/a/b/c/vphone-cli'
make: *** [Makefile:115: setup_machine] Error 2

Changes

• docs: add ipsw install requirement