Try using built-in apt source for gcc-8

58a373e9ed54556e18a13825dfe72d3f7a86b833 and
3868df83330bb897be996604df3862639abbf002
didn't work.

.travis.yml

@@ -85,14 +85,12 @@ env:
 
   - &gcc-8
     compiler: gcc-8
-    # # Not using addon to control retries
-    # addons:
-    #   apt:
-    #     sources:
-    #       - ubuntu-toolchain-r-test
+    addons:
+      apt:
+        sources:
+          - ubuntu-toolchain-r-test
     before_install:
-      - tool/travis_retry.sh sudo -E apt-add-repository -y "ppa:ubuntu-toolchain-r/test"
-      - tool/travis_retry.sh sudo bash -c "${BEFORE_APT_GET_UPDATE}; rm -rf '${TRAVIS_ROOT}/var/lib/apt/lists/'* && exec apt-get update -yq"
+      - tool/travis_retry.sh sudo bash -c "rm -rf '${TRAVIS_ROOT}/var/lib/apt/lists/'* && exec apt-get update -yq"
       - |-
         tool/travis_retry.sh sudo -E apt-get $travis_apt_get_options install \
           gcc-8 \
@@ -352,9 +350,6 @@ env:
     dist: trusty # xenial no longer has ruby-1.9.3
     language: ruby
     rvm: 1.9.3
-    env:
-      # for `apt-get update` error on trusty
-      - BEFORE_APT_GET_UPDATE="apt-get install debian-keyring debian-archive-keyring"
 
   - &x86_64-darwin18
     name: x86_64-darwin18