(build) Update cert subject name + set via env var

This updates the default Authenticode subject name to match what it
changed to during the recent renewal process. It also enables setting
this same name via an Environment Variable so that no changes are needed
urgently if it changes again in the future.

Chocolatey-AU.build.ps1

@@ -31,7 +31,14 @@ param(
     ),
 
     [string]
-    $CertificateSubjectName = "Chocolatey Software, Inc.",
+    $CertificateSubjectName = $(
+        if ($CHOCOLATEY_OFFICIAL_CERT_SUBJECT_NAME) {
+            $CHOCOLATEY_OFFICIAL_CERT_SUBJECT_NAME
+        }
+        else {
+            'Chocolatey Software, Inc'
+        }
+    ),
 
 
     [string]