.gitlab-ci.yml: Move coverage artifacts at the root

They are thus more easily accessible through Gitlab interface
9 jobs for feature-a/coverage-reports in 46 minutes and 19 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test Tools
passed #128545
test_tools

00:00:04

 
  Test Unit
passed #128546
test_algo_unit

00:12:47

passed #128547
test_tools_unit

00:00:04

 
  Test Functional
passed #128548
test_algo_should

00:12:42

 
  Test Shouldvdj
passed #128549
test_algo_shouldvdj

00:17:31

 
  Coverage
passed #128550
algo_coverage

00:00:06

86.5%
 
  Prepare Release
manual #128551
allowed to fail manual
prepare_release
 
  Valgrind Unit
passed #128552
valgrind
algo_valgrind_unit

00:03:06

 
  Valgrind Functional
manual #128553
valgrind allowed to fail manual
algo_valgrind_functional