Azure Pipelines: Retrieve build number from server, pass it to CMake

The script on our server bumps the build number for every new version + commit combination.
Each combination is associated to a unique build number and vice versa.
There's a separate counter for each version.

The reason why we cannot just use "git describe --tags --dirty" is because it relies on the last tag's name and generates a string like "5.01.9674-212-g54280853".
What we want, instead, is the last part of the version to be increased for every build.
Then, once we consider the branch stable enough, we create a tag like "5.01" and bump the version immediately after the new release.

Please note that for pull requests the build number will always be 0, because the secret token is only available in the Nightly pipeline.

.ci/azure-pipelines/linux.yml

@@ -4,12 +4,11 @@ jobs:
   pool:
     vmImage: ubuntu-16.04
   steps:
-  - script: |
-      cd $BUILD_BINARIESDIRECTORY
-      sudo apt -y install cmake gcc g++ ninja-build libncurses5-dev libreadline-dev libssl-dev make zlib1g-dev
-      cmake -G "Ninja" -DCMAKE_BUILD_TYPE=RelWithDebInfo $BUILD_SOURCESDIRECTORY
-      cmake --build .
-      cpack -C Release -G DEB
+  - script: sudo apt-get -y install cmake gcc g++ ninja-build libncurses5-dev libreadline-dev libssl-dev make zlib1g-dev
+    displayName: 'Prepare environment'
+  - script: "$(Build.SourcesDirectory)/.ci/azure-pipelines/linux_build.sh"
+    env:
+      SE_BUILD_NUMBER_TOKEN: $(BUILD_NUMBER_TOKEN)
     displayName: 'Build'
   - script: |
       .ci/appveyor-deb-install-test.sh