Salvage of #48637 (Fixes#48628). On a NixOS-style install the venv's
site-packages lives in the read-only store, so ensure()'s
uv -> pip -> ensurepip ladder spends ~15s bootstrapping ensurepip only
to fail against a target it can never write. Fail fast with an
actionable message pointing at the system package manager.
Retargeted onto current main (the PR's base predates the durable-target
subsystem by ~8.1K commits) with two corrections to the original:
- Gate on _lazy_install_target() is None. The container deployment sets
HERMES_MANAGED=true AND HERMES_LAZY_INSTALL_TARGET (a writable
volume); the original guard would have blocked installs that path
legitimately satisfies, breaking the NixOS-container mode.
- Reason string starts with 'unsupported ' because
refresh_active_features classifies FeatureUnavailable by that prefix;
the original wording made 'hermes update' report a hard failure
instead of a skip.
Placed after _unsupported_feature_reason so a platform-specific reason
(more actionable) wins, and so ensure() agrees with
refresh_active_features, which pre-checks that same function.
liuhao1024