pip >= 23.0 added the PEP 668 --break-system-packages flag. On older pip
builds (e.g. macOS Command Line Tools Python) and on the pip `list`
subcommand of every version, the flag is rejected with
"no such option: --break-system-packages", breaking skill dependency
install/check paths entirely.
- dep_installer: route pip installs through pipRunInstall, which retries
once without the flag when pip rejects it (issue #956)
- pip_update_executor: same retry for the upgrade path
- pip_update_checker: drop the flag from `pip3 list --outdated` - list is
read-only, PEP 668 never applies, and no pip accepts the flag there
- add pip_flags.go helpers + unit tests, and legacy-pip integration tests
reproducing the original failure
Fixes#956