Add secret detection

https://docs.gitlab.com/ee/user/application_security/secret_detection/pipeline/index.html

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

.gitlab-ci.yml

@@ -1,5 +1,6 @@
 stages:
   - build
+  - test
 
 cache: &global_cache
   key:
@@ -17,3 +18,6 @@ build:
     <<: *global_cache
   script:
     - nix --experimental-features 'nix-command flakes' flake check
+
+include:
+  - template: Jobs/Secret-Detection.gitlab-ci.yml