upgrade ubuntu 16.04 (not supported starting dec 2021) => 18.04; revive Linux_i386 (nim-lang#18107)

Author: timotheecour

.gitlab-ci.yml

@@ -1,4 +1,6 @@
-image: ubuntu:16.04
+# xxx unused, out of date
+
+image: ubuntu:18.04
 
 stages:
   - pre-build

azure-pipelines.yml

@@ -19,14 +19,14 @@ jobs:
   strategy:
     matrix:
       Linux_amd64:
-        vmImage: 'ubuntu-16.04'
+        vmImage: 'ubuntu-18.04'
         CPU: amd64
-      # pending bug #17325 (broken again)
-      # Linux_i386:
-      #   # bug #17325: fails on 'ubuntu-16.04' because it now errors with:
-      #   # g++-multilib : Depends: gcc-multilib (>= 4:5.3.1-1ubuntu1) but it is not going to be installed
-      #   vmImage: 'ubuntu-18.04'
-      #   CPU: i386
+      # regularly breaks, refs bug #17325
+      Linux_i386:
+        # on 'ubuntu-16.04' (not supported anymore anyways) it errored with:
+        # g++-multilib : Depends: gcc-multilib (>= 4:5.3.1-1ubuntu1) but it is not going to be installed
+        vmImage: 'ubuntu-18.04'
+        CPU: i386
       OSX_amd64:
         vmImage: 'macOS-10.15'
         CPU: amd64