Bug 1596279 - Install grcov in the Linux coverage opt build to parse gcno/gcda files. r=jmaher

Depends on D52997

Differential Revision: https://phabricator.services.mozilla.com/D52998

--HG--
extra : moz-landing-system : lando

taskcluster/ci/build/linux.yml

@@ -1109,6 +1109,7 @@ linux64-ccov/opt:
             - linux64-sccache
             - linux64-nasm
             - linux64-node
+            - linux64-grcov
 
 linux64-add-on-devel/opt:
     description: "Linux64 add-on-devel"