[CI] Force caching when running test-suite in async mode
[CI] Force caching when running test-suite in async mode

I thought the test-suite infrastructure was always passing
`-async-proofs-cache force`, but in fact it does it only for interactive
tests.

This should speed up the tests quite a bit.

silene/coq,JasonGross/coq,coq/coq,ppedrot/coq,SkySkimmer/coq,gares/coq,Matafou/coq,coq/coq,Zimmi48/coq,gares/coq,coq/coq,Zimmi48/coq,ejgallego/coq,ejgallego/coq,JasonGross/coq,SkySkimmer/coq,JasonGross/coq,SkySkimmer/coq,ejgallego/coq,ppedrot/coq,SkySkimmer/coq,gares/coq,Zimmi48/coq,ppedrot/coq,ppedrot/coq,silene/coq,gares/coq,ejgallego/coq,gares/coq,JasonGross/coq,Matafou/coq,coq/coq,silene/coq,Matafou/coq,Zimmi48/coq,ppedrot/coq