ci: pull before pusing download cache

When other buld jobs updated the build cache, they might be faster
pushing then us. So pull before we push to incorporate their changes
first.

.gitlab-ci.yml

@@ -43,7 +43,7 @@ update:
     - rm -rf gluon/openwrt/dl && mkdir -p gluon/openwrt && ln -s ../../ffs-openwrt-dl-cache gluon/openwrt/dl
   script:
     - make -C gluon GLUON_SITEDIR="$CI_PROJECT_DIR" GLUON_TARGET="$GLUON_TARGET" GLUON_BRANCH=stable FFS_BUILD_DATE=$(cat build.date) V=1 -j`nproc`
-    - cd gluon/openwrt/dl && git add . && git commit -m "update DL cache" && git push || true
+    - cd gluon/openwrt/dl && git add . && git commit -m "update DL cache" && git pull --rebase && git push || true
   except:
     - tags
   artifacts: