fix: Downgrade "running as root" error to a warning by default (#2636)

Currently, by default, rules_python immediately fails when Bazel is run
as root. The reasoning behind this involves .pyc files being generated
for hermetic toolchains when they're first used, causing cache misses;
to work around this, rules_python opts to make the toolchain
installation directory read-only, but running Bazel as root would
circumvent this. So rules_python actively detects if the current user is
root, and hard fails. This check can be disabled by the root module by
setting `python.override(ignore_root_user_error=True)`. (See more
context in the linked issues/PRs.)

This causes a reverberating effect across the Bazel ecosystem, as
rules_python is essentially a dependency of every single Bazel project
through protobuf. Effectively, any Bazel project wishing to run as root
need to add the override tag above, even if they don't have anything to
do with Python at all.

This PR changes the default value of the `ignore_root_user_error` to
True instead. Besides, it now unconditionally tries to make the
toolchain installation directory read-only, and only outputs a warning
if it's detected that the current user is root.

See previous discussions at #713, #749, #907, #1008, #1169, etc.

Fixes https://github.com/bazelbuild/rules_python/issues/1169.

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Co-authored-by: Richard Levasseur <rlevasseur@google.com>