Add a way to override the prerelease label

This is different from `PREVIEW_LABEL`, which is used to control the type of rerelease (such as preview, beta or rc). It will still include the build number from CI. The `PRERELEASE_OVERRIDE` is the entire value after the `-`.

Author: mattleibow

azure-pipelines.yml

@@ -4,6 +4,8 @@ variables:
   XCODE_VERSION: 10.1
   VERBOSITY: minimal
   BUILD_NUMBER: $[counter('$(Build.SourceBranchName)_counter', 1)]
+  PREVIEW_LABEL: 'preview'
+  PRERELEASE_OVERRIDE: ''
 
 resources:
   repositories:

build.cake

@@ -53,6 +53,7 @@ var BUILD_NUMBER = EnvironmentVariable("BUILD_NUMBER") ?? "";
 if (string.IsNullOrEmpty(BUILD_NUMBER)) {
     BUILD_NUMBER = "0";
 }
+var PRERELEASE_OVERRIDE = EnvironmentVariable("PRERELEASE_OVERRIDE") ?? "";
 
 Information ("MONODROID_BASE_PATH: {0}", MONODROID_BASE_PATH);
 Information ("MONODROID_PATH:      {0}", MONODROID_PATH);
@@ -140,7 +141,10 @@ Task("nuget")
 		c.Properties.Add("PackageRequireLicenseAcceptance", new [] { "true" });
 		c.Properties.Add("DesignTimeBuild", new [] { "false" });
 		c.Properties.Add("AndroidSdkBuildToolsVersion", new [] { "28.0.3" });
-		c.Properties.Add("PackageVersionSuffix", new [] { "-" + PREVIEW_LABEL + "." + BUILD_NUMBER });
+		var pre = string.IsNullOrEmpty(PRERELEASE_OVERRIDE)
+			? $"{PREVIEW_LABEL}.{BUILD_NUMBER}"
+			: $"{PRERELEASE_OVERRIDE}"
+		c.Properties.Add("PackageVersionSuffix", new [] { "-" + pre });
 	});
 
 	var xmlns = (XNamespace)"http://schemas.microsoft.com/developer/msbuild/2003";