chore(tooling): Relax requirement on updated preview for the sake of CI

This didn't sed to be a problem because of a race condition in Fontship
that resulted in the rule being discarded anyway. When we fixed that bug
(using double expansion and a POSTFONTSHIPEVAL) we introduced
a dependency since we don't have XeLaTeX available in CI. This makes it
possible to regenerate locally but not required to generate packages.

fontship.mk

@@ -30,7 +30,7 @@ endef
 
 define POSTFONTSHIPEVAL =
 
-$$(DOCSDIR)/preview.pdf: $$(DOCSDIR)/preview.tex $$(STATICOTFS) | $$(BUILDDIR)
+$$(DOCSDIR)/preview.pdf: $$(DOCSDIR)/preview.tex | $$(STATICOTFS) $$(BUILDDIR)
 	xelatex --interaction=batchmode -output-directory=$$(BUILDDIR) $$<
 	cp $(BUILDDIR)/$$(@F) $$@