fix(NODE-5600): use ubuntu 18 to build and publish (#162)

Author: durran

.evergreen/config.yml

@@ -163,23 +163,23 @@ buildvariants:
     tasks:
       - run-prebuild
       - run-prebuild-force-publish
-  - name: ubuntu2004-64
-    display_name: 'Ubuntu 20.04 64-bit'
-    run_on: ubuntu2004-small
+  - name: ubuntu1804-64
+    display_name: 'Ubuntu 18.04 64-bit'
+    run_on: ubuntu1804-test
     expansions:
       has_packages: true
-      packager_distro: ubuntu2004
+      packager_distro: ubuntu1804
       packager_arch: x86_64
     tasks:
       - run-tests-ubuntu
       - run-prebuild
       - run-prebuild-force-publish
   - name: ubuntu1804-arm64
-    display_name: 'Ubuntu 20.04 arm64'
-    run_on: ubuntu2004-arm64-small
+    display_name: 'Ubuntu 18.04 arm64'
+    run_on: ubuntu1804-arm64-build
     expansions:
       has_packages: true
-      packager_distro: ubuntu2004
+      packager_distro: ubuntu1804
       packager_arch: arm64
     tasks:
       - run-tests-ubuntu