prefill: fix ca_file to actually expect file and not a directory

This was leftover from an old version which used directory with
certificates instead of bundle in single file.

Related: !511
18 jobs for prefill_fix in 8 minutes and 13 seconds (queued for 10 seconds)
Status Job ID Name Coverage
  Build
passed #135708
docker linux amd64
build:clang:linux:amd64

00:00:40

passed #135707
docker linux amd64
build:linux:amd64

00:00:38

passed #135709
docker linux amd64 allowed to fail
srpm

00:00:25

 
  Test
passed #135716
docker linux amd64
deckard:linux:amd64

00:07:04

passed #135723
privileged
distro:epel-7

00:02:59

passed #135722
privileged
distro:fedora-27

00:04:08

passed #135715
docker
doc

00:00:49

passed #135714
docker linux amd64
installcheck:linux:amd64

00:01:04

passed #135717
docker linux amd64
installcheck:valgrind:linux:amd64

00:01:39

passed #135711
docker
lint:c

00:02:35

failed #135712
docker allowed to fail
lint:clang-scan-build

00:01:16

passed #135710
docker
lint:lua

00:00:37

passed #135720
docker linux amd64
respdiff:iter:tcp:linux:amd64

00:03:45

passed #135721
docker linux amd64
respdiff:iter:tls:linux:amd64

00:04:09

passed #135719
docker linux amd64
respdiff:iter:udp:linux:amd64

00:03:47

passed #135713
docker linux amd64
test:linux:amd64

00:01:24

passed #135718
docker linux amd64
test:linux:amd64:valgrind

00:01:17

 
  Coverage
passed #135724
docker linux amd64
coverage

00:00:23

69.1%
 
Name Stage Failure
failed
lint:clang-scan-build Test
                                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
5 warnings generated.
CCLD daemon/kresd
scan-build: 28 bugs found.
scan-build: Run 'scan-view /builds/knot/knot-resolver/scan-results/2018-05-09-164742-14-1' to examine bug reports.
Uploading artifacts...
scan-results: found 33 matching files
Uploading artifacts to coordinator... ok
id=135712 responseStatus=201 Created token=UG6MoZhQ
ERROR: Job failed: exit code 1