gitlab-ci: Fix 'apt' warnings on crossbuilds by seting TERM and DEBIAN_FRONTEND

Seems that Debian Testing 'apt' defaults to 'dialog' mode by default, which
produces warnings since the Docker image don't come with 'noninteractive' set.

These warnings are inoffensive but they pollute the job output. Indeed,
they were making some GIMPers a bit worried on IRC.

---

Also, added a comment about why we always use Debian Testing for crossbuilds.

.gitlab-ci.yml

@@ -429,8 +429,12 @@ gimp-flatpak-x64:
     - <<: *CI_COMMIT
     - if: '$GIMP_CI_CROSSROAD_WIN64 != null'
   variables:
-    DEB_VERSION: testing
     MSYSTEM_PREFIX: clang64
+    #To match Debian Testing compiler with MSYS2 compiler resources
+    DEB_VERSION: testing
+    #To avoid noisy 'apt' warnings
+    DEBIAN_FRONTEND: noninteractive
+    TERM: dumb
 
 image-debian-x64-cross:
   extends: .win-cross