Bug 1600735 - Make debug fuzzing builds use gcc 8. r=froydnj

GCC8 happens not to generate the code that causes the crash, so do that for now
to unblock fuzzers from hitting this.

We still need to figure out what to do about the more general issue of course...

Differential Revision: https://phabricator.services.mozilla.com/D55985

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

taskcluster/ci/build/linux.yml

@@ -106,7 +106,7 @@ linux64-fuzzing/debug:
     fetches:
         toolchain:
             - linux64-clang
-            - linux64-gcc
+            - linux64-gcc-8
             - linux64-cbindgen
             - linux64-sccache
             - linux64-rust