docs/ci: allow overriding sphinx builder

Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19574>

.gitlab-ci.yml

@@ -128,6 +128,8 @@ stages:
 
 
 .docs-base:
+  variables:
+    BUILDER: html
   extends:
     - .fdo.ci-fairy
     - .build-rules
@@ -135,7 +137,7 @@ stages:
   - apk --no-cache add graphviz doxygen
   - pip3 install sphinx===5.1.1 breathe===4.34.0 mako===1.2.3 sphinx_rtd_theme===1.0.0
   - docs/doxygen-wrapper.py --out-dir=docs/doxygen_xml
-  - sphinx-build -W -b html docs public
+  - sphinx-build -W -b $BUILDER docs public
 
 pages:
   extends: .docs-base