Fix empty XDG_CACHE_HOME Behavior

Use the default $HOME/.cache one for empty XDG_CACHE_HOME.

Fixes fontconfig/fontconfig#243
3 jobs for issue/243 in 6 minutes and 2 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Build
passed #3600084
mingw-build

00:05:50

passed #3600082
shared-build

00:04:45

passed #3600083
static-build

00:03:59