ci: install gcovr via apt

Change-Id: I017ec6e4281a0348eca0361643cb3af0167c420f
Refs: #4771
diff --git a/.jenkins.d/README.md b/.jenkins.d/README.md
index a69ef9e..aeb94b0 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
 
 - `WAF_JOBS`: number of parallel build jobs used by waf, defaults to 1.