.azure-pipelines/release.yml: add pre-build stage

Add a stage to the Azure Pipelines release pipeline that can perform
pre-build validation, and capture information used in all subsequent
stages and jobs. In this case, the VERSION file contents.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>

Author: mjcheetham

.azure-pipelines/release.yml

@@ -25,14 +25,22 @@ extends:
         image: win-x86_64-ado1es
         os: windows
     stages:
-      - stage: windows
-        displayName: 'Windows'
+      - stage: prebuild
+        displayName: 'Pre-build Validation'
         jobs:
-          - job: win_x86_build
-            displayName: 'Windows Build and Sign (x86)'
+          - job: vars
+            displayName: 'Read variables'
             pool:
               name: GitClient-1ESHostedPool-intel-pc
-              image: win-x86_64-ado1es
-              os: windows
+              image: ubuntu-x86_64-ado1es
+              os: linux
             steps:
               - checkout: self
+              - task: Bash@3
+                displayName: 'Read version file'
+                name: version
+                inputs:
+                  targetType: inline
+                  script: |
+                    # Set the version variable for later stages to consume
+                    echo "##vso[task.setvariable variable=value;isOutput=true;isReadOnly=true]$(cat ./VERSION | sed -E 's/.[0-9]+$//')"