ugh, apparently bug in gitlab-runner, try workaround

.gitlab-ci.yml

@@ -4,12 +4,16 @@ stages:
   - build
 
 cache:
+  key: apt-cache
   paths:
     - apt-cache/
 
+before_script:
+  - export APT_CACHE_DIR=`pwd`/apt-cache && mkdir -pv $APT_CACHE_DIR
+  - sudo apt-get update -yq && sudo apt-get -o dir::cache::archives="$APT_CACHE_DIR" install -y build-essential cmake
+
 build:
   stage: build
   script:
-    - apt update && apt -o dir::cache::archives="apt-cache" install -y build-essential cmake
     - gcc --version
     - mkdir -p build && cd build && cmake -DCMAKE_BUILD_TYPE=Release .. && make -j