After a recent change, `piglit-traces.sh` automatically sets the caching
proxy, so update the docs to reflect this.
Also update the name of the variable from `FDO_HTTP_CACHE_URI` to
`LAVA_HTTP_CACHE_URI`.
Fixes: fa74e939bf ("ci/piglit: automatically use LAVA proxy")
Signed-off-by: Valentine Burley <valentine.burley@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37862>