Make build task cred provider not cachable (#39)

Author: Satu Bailey

Build.props

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup>
-    <CredentialProviderVersion>0.1.5</CredentialProviderVersion>
+    <CredentialProviderVersion>0.1.6</CredentialProviderVersion>
   </PropertyGroup>
 </Project>

CredentialProvider.Microsoft/CredentialProviders/VstsBuildTask/VstsBuildTaskCredentialProvider.cs

@@ -25,6 +25,8 @@ public VstsBuildTaskCredentialProvider(ILogger logger)
 
         protected override string LoggingName => nameof(VstsBuildTaskCredentialProvider);
 
+        public override bool IsCachable { get { return false; } }
+
         public override Task<bool> CanProvideCredentialsAsync(Uri uri)
         {
             string uriPrefixesString = Environment.GetEnvironmentVariable(EnvUtil.BuildTaskUriPrefixes);