Linux one-click-start.sh and setup_backend_linux.sh previously installed
dependencies from the default PyPI source, which is often slow or blocked
in mainland China. Mirror the Windows pipeline's PyPI mirror behaviour by
probing well-known mirrors and falling back to the official source.
- New scripts/pypi_mirror.sh: ordered list of Tsinghua / Aliyun / USTC /
Tencent / pypi.org; pypi_mirror_probe checks each with curl or wget
(3s timeout); pypi_mirror_select returns the first reachable URL;
ensure_pypi_mirror_pip_args sets PIP_ARGS=("-i" <url>) only when the
caller has not already supplied --pip-arg. Reusable via 'source'.
- one-click-start.sh sources pypi_mirror.sh and calls
ensure_pypi_mirror_pip_args before invoking the backend installer, so a
bare one-click-start.sh on a CN host picks the fastest mirror.
- scripts/setup_backend_linux.sh does the same for direct invocations
(e.g. ./scripts/setup_backend_linux.sh), with the same skip-if-pip-arg-
set semantics. Header comment updated to document the behaviour.
Users who pass --pip-arg keep full control; everyone else gets a working
mirror automatically and the install no longer wedges on pypi.org.
