ci: Fix regex for reporting code coverage

We are now using gcovr instead of lcov after
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=987818
has been closed.

The difference in output format tripped up reported code coverage in CI.

.gitlab-ci.yml

@@ -47,7 +47,7 @@ test:native:
     - xvfb-run -s -noreset ninja -C _build test
     - cp _build/src/libcalls-vala.a.p/*.c _build
     - ninja -C _build coverage
-  coverage: '/^\s+lines\.+:\s+([\d.]+\%)\s+/'
+  coverage: '/^lines:\s+([\d.]+\%)\s+/'
 
 build-gtkdoc:
   extends: build:native