Bug 1744890 - Use clang 13 for mar-tools toolchain builds. r=firefox-build-system-reviewers,andi

Differential Revision: https://phabricator.services.mozilla.com/D133179

taskcluster/ci/toolchain/misc.yml

@@ -74,7 +74,7 @@ linux64-mar-tools:
         symbol: TL(mar-tools)
     fetches:
         toolchain:
-            - linux64-clang-7
+            - linux64-clang-13
             - linux64-binutils
     run:
         script: build-mar-tools.sh