Only write highlight to cache when CacheDir is set
Only write highlight to cache when CacheDir is set

To avoid writing cache files when testing.

MarkDBlackwell/hugo,tatsushid/hugo,vbatoufflet/hugo,mdhender/hugo,digitalcraftsman/hugo,mdhender/hugo,tischda/hugo,anthonyfok/hugo,soundrussian/hugo,ffrizzo/hugo,sprig/hugo,bramp/hugo,fedelibre/hugo,bogem/hugo,badoet/hugo,tjyang/hugo,jygastaud/hugo,fedelibre/hugo,fhergomez/hugo,ifraixedes/hugo,abourget/hugo,mdhender/hugo,MarkDBlackwell/hugo,anthonyfok/hugo,bogem/hugo,badoet/hugo,halostatue/hugo,mdhender/hugo,bramp/hugo,tubo028/hugo,soundrussian/hugo,halostatue/hugo,derekperkins/hugo,tubo028/hugo,fedelibre/hugo,ryanclarke/hugo,kujohn/hugo,ryanclarke/hugo,Zenithar/hugo,ifraixedes/hugo,kujohn/hugo,digitalcraftsman/hugo,mayah/hugo,sprig/hugo,melo/hugo,fhergomez/hugo,mitchchn/hugo,ffrizzo/hugo,Zenithar/hugo,abourget/hugo,melo/hugo,vbatoufflet/hugo,tatsushid/hugo,jygastaud/hugo,tischda/hugo,coderzh/hugo,gohugoio/hugo,coderzh/hugo,fedelibre/hugo,tjyang/hugo,mitchchn/hugo,gohugoio/hugo,derekperkins/hugo,mayah/hugo,anthonyfok/hugo,gohugoio/hugo