[modal ci] fixes (#7676)
1. `modal-accelerate` needs now `uv` installed explicitly since the
image change to 2025 one.
2. moved accelerate repo cloning into the job, since the original way
was incorrect as it was caching some accelerate version and not updating
it.
3. annotated that how to actually test the ci work when changing the
workflow as `pull_request_target` will not run the updated .py+.yaml
files.
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Signed-off-by: Stas Bekman <stas@stason.org>
