Bug 1669170 - For Coverity static-analysis use clang-10 and not default clang versions. r=marco

Differential Revision: https://phabricator.services.mozilla.com/D92377

taskcluster/ci/source-test/coverity.yml

@@ -21,7 +21,7 @@ job-defaults:
         tooltool-downloads: public
     fetches:
         toolchain:
-            - linux64-clang
+            - linux64-clang-10
             - linux64-rust
             - linux64-cbindgen
             - linux64-nasm