ci: Add a regex to parse line coverage statistics from genhtml output

GitLab can then use this to annotate each pipeline with its code
coverage statistics. It can only use one figure, so we choose lines
(rather than function or branch coverage) since it’s the most intuitive
figure.

This parses the ‘lines’ line from output like:

Overall coverage rate:
  lines......: 76.7% (108959 of 142122 lines)
  functions..: 80.7% (10294 of 12763 functions)
  branches...: 51.3% (50226 of 97953 branches)

Signed-off-by: Philip Withnall <withnall@endlessm.com>

.gitlab-ci.yml

@@ -50,6 +50,7 @@ coverage:
       - _coverage/
   script:
     - bash -x ./.gitlab-ci/coverage-docker.sh
+  coverage: '/^\s+lines\.+:\s+([\d.]+\%)\s+/'
 
 pages:
   stage: deploy