Fix CI for clang-6 when cross-compiled.
diff --git a/ci/build.linux.gitlab-ci.yml b/ci/build.linux.gitlab-ci.yml
index 1217fbf..4a7897d 100644
--- a/ci/build.linux.gitlab-ci.yml
+++ b/ci/build.linux.gitlab-ci.yml
@@ -61,7 +61,7 @@
   extends: .build:linux:cross:x86-64
   image: ubuntu:20.04
   variables:
-    EIGEN_CI_INSTALL: clang-6.0 lld-6.0
+    EIGEN_CI_INSTALL: g++-8 clang-6.0 lld-6.0
     EIGEN_CI_C_COMPILER: clang-6.0
     EIGEN_CI_CXX_COMPILER: clang++-6.0
     EIGEN_CI_CROSS_INSTALL: g++-8-x86-64-linux-gnu clang-6.0 lld-6.0
diff --git a/ci/test.linux.gitlab-ci.yml b/ci/test.linux.gitlab-ci.yml
index 6d1c7e9..590bdee 100644
--- a/ci/test.linux.gitlab-ci.yml
+++ b/ci/test.linux.gitlab-ci.yml
@@ -98,7 +98,7 @@
   image: ubuntu:20.04
   needs: [ build:linux:cross:x86-64:clang-6:default ]
   variables:
-    EIGEN_CI_INSTALL: clang-6.0 lld-6.0
+    EIGEN_CI_INSTALL: g++-8 clang-6.0 lld-6.0
 
 test:linux:x86-64:clang-6:default:official:
   extends: .test:linux:x86-64:clang-6:default