.gitlab-ci.yml: Switch to apt-get

Because of messages warning about apt CLI instability

.gitlab-ci.yml

 image: debian:testing-slim
 
 before_script:
-  - apt update && apt install -y libhts3 zlib1g libsdsl3
+  - apt-get update && apt-get install -y libhts3 zlib1g libsdsl3
 
 build:
   stage: build
   before_script:
-    - apt update && apt install -y make g++ libsdsl-dev libhts-dev zlib1g-dev
+    - apt-get update && apt-get install -y make g++ libsdsl-dev libhts-dev zlib1g-dev
   script:
     - make
     - make -C tests
@@ -18,7 +18,7 @@ build:
 test:
   stage: test
   before_script:
-    - apt update && apt install -y zutils diffutils
+    - apt-get update && apt-get install -y zutils diffutils
   script:
     - cd tests
     - ./test_bam