ci: move coverity in its own pipeline

Since Coverity is down for a unspecified timeframe, isolate it from the
"hosted" nightlies.

azure-pipelines/coverity.yml

+resources:
+- repo: self
+
+- job: coverity
+  displayName: 'Coverity'
+  pool:
+    vmImage: 'Ubuntu 16.04'
+  steps:
+  - task: Docker@0
+    displayName: Build
+    inputs:
+      action: 'Run an image'
+      imageName: 'libgit2/trusty-openssl:latest'
+      volumes: |
+       $(Build.SourcesDirectory):/src
+       $(Build.BinariesDirectory):/build
+      envVars: |
+       COVERITY_TOKEN=$(COVERITY_TOKEN)
+      workDir: '/build'
+      containerCommand: '/src/ci/coverity.sh'
+      detached: false

azure-pipelines/nightly.yml

@@ -173,22 +173,3 @@ jobs:
        CC=gcc
        CMAKE_OPTIONS=-DUSE_HTTPS=OpenSSL
        SKIP_PROXY_TESTS=true
-
-- job: coverity
-  displayName: 'Coverity'
-  pool:
-    vmImage: 'Ubuntu 16.04'
-  steps:
-  - task: Docker@0
-    displayName: Build
-    inputs:
-      action: 'Run an image'
-      imageName: 'libgit2/trusty-openssl:latest'
-      volumes: |
-       $(Build.SourcesDirectory):/src
-       $(Build.BinariesDirectory):/build
-      envVars: |
-       COVERITY_TOKEN=$(COVERITY_TOKEN)
-      workDir: '/build'
-      containerCommand: '/src/ci/coverity.sh'
-      detached: false