gitlab-ci: Fix doc-tarball job

Previously we used the deb9-debug job which used the `validate` build
flavour which disabled `BUILD_SPHINX_PDF`. Fix this.

Fixes #16890.

.gitlab-ci.yml

@@ -503,6 +503,8 @@ validate-x86_64-linux-deb9-debug:
   stage: build
   variables:
     BUILD_FLAVOUR: validate
+    # Override validate flavour default; see #16890.
+    BUILD_SPHINX_PDF: "YES"
     TEST_TYPE: slowtest
     TEST_ENV: "x86_64-linux-deb9-debug"
   artifacts:

mk/flavours/validate.mk

@@ -6,8 +6,8 @@ GhcLibHcOpts       = -O -dcore-lint -dno-debug-output
 BUILD_PROF_LIBS    = NO
 SplitSections      = NO
 HADDOCK_DOCS       = YES
-BUILD_SPHINX_HTML  = YES
-BUILD_SPHINX_PDF   = NO
+BUILD_SPHINX_HTML ?= YES
+BUILD_SPHINX_PDF  ?= NO
 
 ifeq "$(ValidateHpc)" "YES"
 GhcStage2HcOpts   += -fhpc -hpcdir $(TOP)/testsuite/hpc_output/