[CI] Downgrade ci_cpu llvm version back to 11

[neildhickey]

The upgrade to llvm 15 caused failures in topi tests python/topi/python/test_topi_math.py::test_cast[float64-float16-llvm]. It seems to be a miscompilation due to opaque pointers. Downgrading while working to fix the issue.

[tvm-bot]

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• cc @Mousius, @areusch, @driazati, @gigiblender, @leandron _{See #10317 for details}

_{Generated by tvm-bot}

[ekalda]

@tvm-bot rerun

[yongwww]

@neildhickey thanks for the investigation! I saw this issue in my ci upgrade work (#14651), it would better to fix this cast issue, looks the cast from f64 to f16 generates wrong result.

[neildhickey]

@yongwww Yes I do want to fix the issue properly but put up this patch as a temporary work around until I am able to fix it properly

[leandron]

Thanks for looking into it @neildhickey

[leandron]

Merging this to make the current ci_cpu image sources to work again.