gitlab-ci: try to make the Debian build less noisy.

See whether setting DH_QUIET to 1 and MAKEFLAGS to --silent causes the
build process not to print the compile command, complete with a big list
of compiler options, for every single file being built, in the hopes of
producing less than the limit of 4MB of log text.

.gitlab-ci.yml

@@ -184,6 +184,8 @@ build:debian-stable:
   script:
     # Shared GitLab runners limit the log size to 4M, so reduce verbosity. See
     # https://gitlab.com/gitlab-com/support-forum/issues/2790
+    - export DH_QUIET=1
+    - export MAKEFLAGS=--silent
     - dpkg-buildpackage -b --no-sign -jauto
     - mkdir debian-packages
     - mv ../*.deb debian-packages/