CONTRIB: pin spcx-plugin to v0.2.x branch instead of master (v1.5.0 release) (#2014)

## What?
Change EXT plugin ref to v0.2.x release branch

## Why?
Currently, it's pinned to master which aligned with UCX master while
NIXL release uses v1.22.x,
v0.2.x branch of the plugin is aligned with UCX v1.22.x plugin API

<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **Bug Fixes**
* Updated the default UCX SPCX plugin version used during container
builds to the v0.2.x release line.
* Build help and options now reflect the updated default when no version
is specified.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Roie Danino <rdanino@nvidia.com>