switch GITHUB_URL to GITHUB_SERVER_URL (#248)

src/url-helper.ts

@@ -20,5 +20,10 @@ export function getFetchUrl(settings: IGitSourceSettings): string {
 }
 
 export function getServerUrl(): URL {
-  return new URL(process.env['GITHUB_URL'] || 'https://github.com')
+  // todo: remove GITHUB_URL after support for GHES Alpha is no longer needed
+  return new URL(
+    process.env['GITHUB_SERVER_URL'] ||
+      process.env['GITHUB_URL'] ||
+      'https://github.com'
+  )
 }