Bug 1471339 - Use Clang 7 for opt Linux ccov builds too. r=me

--HG--
extra : rebase_source : 75edec2c4ccccee9864dfe22dccf5c798841a861

taskcluster/ci/build/linux.yml

@@ -1000,7 +1000,7 @@ linux64-ccov/opt:
         tooltool-downloads: public
         need-xvfb: true
     toolchains:
-        - linux64-clang
+        - linux64-clang-7
         - linux64-rust
         - linux64-gcc
         - linux64-sccache