installer: Fix CA Cert file extension
installer: Fix CA Cert file extension

This fixes installing the cert on Windows. Other systems are able to
determine the file type based on the file contents, Windows does not.

Signed-off-by: Jesse Stuart <a5c95b3d7cb4d0ae05a15c79c79ab458dc2c8f9e@jessestuart.ca>

benbjohnson/flynn,lmars/flynn,benbjohnson/flynn,benbjohnson/flynn,benbjohnson/flynn,flynn/flynn,benbjohnson/flynn,flynn/flynn,philiplb/flynn,lmars/flynn,philiplb/flynn,flynn/flynn,philiplb/flynn,lmars/flynn,lmars/flynn,philiplb/flynn,flynn/flynn,lmars/flynn,philiplb/flynn,flynn/flynn,benbjohnson/flynn