fix(qwen-tts): install flash-attn on cuda13 images (#9293)

The cuda12 image installs flash-attn via requirements-cublas12-after.txt,
but there was no cublas13 equivalent, so cuda13-qwen-tts never installed
flash_attn and fell back to SDPA with warnings. Add the matching
requirements-cublas13-after.txt.

Assisted-by: claude:claude-opus-4-8 [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
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