coverity.yml: use environment variable for INCLUDE_DIAGNOSTICS value

let's use $COVERITY_ENABLE_DIAGNOSTICS environment variable as a value for INCLUDE_DIAGNOSTICS argument.
so we can easily change it when needed without code modification.

Closes: PCF-2037
References: PCF-1795

Signed-off-by: Mouna Aloui <mouna.aloui@softathome.com>

.gitlab/coverity.yml

@@ -18,7 +18,7 @@ include:
     BRIDGE_COVERITY_LOCAL: true
     BRIDGE_BREAK: false
     BRIDGE_COVERITY_CLEAN_COMMAND: make clean
-    # INCLUDE_DIAGNOSTICS: 'true'
+    INCLUDE_DIAGNOSTICS: $COVERITY_ENABLE_DIAGNOSTICS
   rules:
     - if: $CI_COMMIT_REF_PROTECTED && $CI_COMMIT_TAG
       variables: