ci: install gcovr via apt

Change-Id: I83a16e080145e206a08cc5255a906e41605ceca8
refs: #4771
diff --git a/.jenkins.d/README.md b/.jenkins.d/README.md
index 39fa40b..5813349 100644
--- a/.jenkins.d/README.md
+++ b/.jenkins.d/README.md
@@ -27,7 +27,7 @@
   Possible values:
 
   * empty: default build process
-  * `code-coverage` (Linux OS is assumed): debug build with tests and code coverage analysis
+  * `code-coverage` (Ubuntu Linux is assumed): debug build with tests and code coverage analysis
   * `limited-build`: only a single debug build with tests
 
 - `CACHE_DIR`: the variable defines a path to folder containing cached files from previous builds,