ci: Run scan-build in a different build directory

The build artifacts from earlier jobs in the pipeline all use the
`_build` directory. When they are copied in to the scan-build job, they
are probably marked as read-only. This means that the `meson scan-build`
run can’t write to `_build/meson-logs/meson-log.txt` and fails.

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

.gitlab-ci.yml

@@ -255,13 +255,13 @@ scan-build:
             -Ddtrace=true
             -Dfam=true
             -Dinstalled_tests=true
-            _build
-    - ninja -C _build scan-build
+            _scan_build
+    - ninja -C _scan_build scan-build
   artifacts:
     name: "glib-${CI_JOB_NAME}-${CI_COMMIT_REF_NAME}"
     when: always
     paths:
-      - "_build/meson-logs/scanbuild"
+      - "_scan_build/meson-logs/scanbuild"
 
 pages:
   stage: deploy