Bug in root module detection

[gleyba]

In buf/extensions.bzl, there are such lines:

            if toolchains.name != _DEFAULT_TOOLCHAIN_NAME and not mod.is_root:
                fail("""\
                Only the root module may override the default name for the buf toolchains.
                This prevents conflicting registrations in the global namespace of external repos.
                """)

I guess it should be

            if toolchains.name == _DEFAULT_TOOLCHAIN_NAME and not mod.is_root:
                fail("""\
                Only the root module may override the default name for the buf toolchains.
                This prevents conflicting registrations in the global namespace of external repos.
                """)

There is already protovalidate non-root module overriding default toolchain version
https://github.com/bufbuild/protovalidate/blob/main/MODULE.bazel#L37