Use latest ca-certificates for ffsend binary test against public instance

.gitlab-ci.yml

@@ -136,7 +136,8 @@ test-public:
   variables:
     GIT_STRATEGY: none
     RUST_TARGET: x86_64-unknown-linux-musl
-  before_script: []
+  before_script:
+    - apk add ca-certificates
   script:
     # Prepare ffsend binary, create random file
     - mv ./ffsend-$RUST_TARGET ./ffsend