ci: use bionic for non-amd64 builds

Use Bionic so that we have a modern libssh2 (for communicating with
GitHub).  We've ported fixes to our Trusty-based amd64 images, but
maintaining patches for multiple platforms is heinous.

azure-pipelines/nightly.yml

@@ -2,57 +2,57 @@ resources:
 - repo: self
 
 jobs:
-- job: linux_x86_xenial_gcc_openssl
-  displayName: 'Linux (x86; Xenial; GCC; OpenSSL)'
+- job: linux_x86_bionic_gcc_openssl
+  displayName: 'Linux (x86; Bionic; GCC; OpenSSL)'
   pool:
     vmImage: 'Ubuntu 16.04'
   steps:
   - template: docker.yml
     parameters:
       qemu: 'true'
-      imageName: 'libgit2/xenial-x86:test'
+      imageName: 'libgit2/bionic-x86:test'
       environmentVariables: |
        CC=gcc
        CMAKE_OPTIONS=-DUSE_HTTPS=OpenSSL
        LEAK_CHECK=valgrind
 
-- job: linux_x86_xenial_clang_openssl
-  displayName: 'Linux (x86; Xenial; Clang; OpenSSL)'
+- job: linux_x86_bionic_clang_openssl
+  displayName: 'Linux (x86; Bionic; Clang; OpenSSL)'
   pool:
     vmImage: 'Ubuntu 16.04'
   steps:
   - template: docker.yml
     parameters:
       qemu: 'true'
-      imageName: 'libgit2/xenial-x86:test'
+      imageName: 'libgit2/bionic-x86:test'
       environmentVariables: |
        CC=clang
        CMAKE_OPTIONS=-DUSE_HTTPS=OpenSSL
        LEAK_CHECK=valgrind
 
-- job: linux_arm32_xenial_gcc_openssl
-  displayName: 'Linux (arm32; Xenial; GCC; OpenSSL)'
+- job: linux_arm32_bionic_gcc_openssl
+  displayName: 'Linux (arm32; Bionic; GCC; OpenSSL)'
   pool:
     vmImage: 'Ubuntu 16.04'
   steps:
   - template: docker.yml
     parameters:
       qemu: 'true'
-      imageName: 'libgit2/xenial-arm32:test'
+      imageName: 'libgit2/bionic-arm32:test'
       environmentVariables: |
        CC=gcc
        CMAKE_OPTIONS=-DUSE_HTTPS=OpenSSL
        SKIP_PROXY_TESTS=true
 
-- job: linux_arm64_xenial_gcc_openssl
-  displayName: 'Linux (arm64; Xenial; GCC; OpenSSL)'
+- job: linux_arm64_bionic_gcc_openssl
+  displayName: 'Linux (arm64; Bionic; GCC; OpenSSL)'
   pool:
     vmImage: 'Ubuntu 16.04'
   steps:
   - template: docker.yml
     parameters:
       qemu: 'true'
-      imageName: 'libgit2/xenial-arm64:test'
+      imageName: 'libgit2/bionic-arm64:test'
       environmentVariables: |
        CC=gcc
        CMAKE_OPTIONS=-DUSE_HTTPS=OpenSSL