Improve GitHub cache documentation

When using `buildctl`, it's not explained in the documentation that
GitHub attributes `url` and `token` must be manually set to make this works.
That miscomprehension is visible with #2706.

To do not meet that error again, this commit add a precision on how
use GitHub cache with buildctl.

Signed-off-by: Vasek - Tom C <tom.chauveau@epitech.eu>

README.md

@@ -414,6 +414,8 @@ Following attributes are required to authenticate against the [Github Actions Ca
 where `url` and `token` will be automatically set. To use this backend in a inline `run` step, you have to include [crazy-max/ghaction-github-runtime](https://github.com/crazy-max/ghaction-github-runtime)
 in your workflow to expose the runtime.
 
+:warning: If used directly with `buildctl` CLI, you must inline `url` and `token` attributes. For instance : `type=gha,url=$ACTIONS_CACHE_URL,token=$ACTIONS_RUNTIME_TOKEN`
+
 `--export-cache` options:
 * `type=gha`
 * `mode=min` (default): only export layers for the resulting image