ci: Move validate-x86_64-linux-deb9 to full-build stage

The `build` stage is meant to be a minimal smoke test to weed out broken
commits. The `validate-x86_64-linux-deb9` build will generally catch a
subset of issues caught by `validate-x86_64-linux-deb9-debug` so only
the latter should be in `build`.

.gitlab-ci.yml

@@ -286,7 +286,7 @@ nightly-i386-linux-deb9:
 
 .build-x86_64-linux-deb9:
   extends: .validate-linux
-  stage: build
+  stage: full-build
   image: "registry.gitlab.haskell.org/ghc/ci-images/x86_64-linux-deb9:$DOCKER_REV"
   variables:
     TEST_ENV: "x86_64-linux-deb9"
@@ -313,6 +313,7 @@ nightly-x86_64-linux-deb9:
 # N.B. Has DEBUG assertions enabled in stage2
 validate-x86_64-linux-deb9-debug:
   extends: .build-x86_64-linux-deb9
+  stage: build
   variables:
     BUILD_FLAVOUR: validate
     TEST_ENV: "x86_64-linux-deb9-debug"