fix(GithubCredentialsProvider): support selective app installs with GHE hosts

Signed-off-by: Phil Kuang <pkuang@factset.com>

.changeset/khaki-hotels-doubt.md

@@ -0,0 +1,5 @@
+---
+'@backstage/integration': patch
+---
+
+Support selective GitHub app installation for GHE

packages/integration/src/github/GithubCredentialsProvider.ts

@@ -64,12 +64,14 @@ const HEADERS = {
  */
 class GithubAppManager {
   private readonly appClient: Octokit;
+  private readonly baseUrl?: string;
   private readonly baseAuthConfig: { appId: number; privateKey: string };
   private readonly cache = new Cache();
   private readonly allowedInstallationOwners: string[] | undefined; // undefined allows all installations
 
   constructor(config: GithubAppConfig, baseUrl?: string) {
     this.allowedInstallationOwners = config.allowedInstallationOwners;
+    this.baseUrl = baseUrl;
     this.baseAuthConfig = {
       appId: config.appId,
       privateKey: config.privateKey,
@@ -108,6 +110,7 @@ class GithubAppManager {
       });
       if (repo && result.data.repository_selection === 'selected') {
         const installationClient = new Octokit({
+          baseUrl: this.baseUrl,
           auth: result.data.token,
         });
         const repos = await installationClient.paginate(