[coq_makefile] Quote `OCAMLFIND` everywhere
[coq_makefile] Quote `OCAMLFIND` everywhere

We already quote it in some places; it needs to be quoted everywhere to
support Windows and paths with spaces.

Fixes #15664

ejgallego/coq,SkySkimmer/coq,coq/coq,Zimmi48/coq,Zimmi48/coq,coq/coq,coq/coq,coq/coq,Zimmi48/coq,ejgallego/coq,SkySkimmer/coq,SkySkimmer/coq,ejgallego/coq,SkySkimmer/coq,Zimmi48/coq,ejgallego/coq