Pass correct baseUrl to octokit

The PR #1246 replaced the `getOctokit` method from the
`octokit-provider.ts` file with the `getOctokit` method from the
`@actions/github` package.
The octokit-provider was previously responsible for creating an Octokit
instance and setting the `baseUrl` via the `getServerApiUrl` helper
function. This function calls `getServerUrl` which reads the server url
from the `GITHUB_SERVER_URL` environment variable, which on GHES is set
to the enterprise instance.
This commit restores the previous behaviour by calling `getServerApiUrl`
in all places where an octokit instance is created.

Co-authored-by: Markus Wolf <mail@markus-wolf.de>

__test__/github-api-helper.test.ts

@@ -0,0 +1,37 @@
+import * as github from '@actions/github'
+import * as githubApiHelper from '../lib/github-api-helper'
+
+jest.mock('@actions/github')
+
+describe('github-api-helper tests', () => {
+  describe('github enterprise compatibility', () => {
+    beforeEach(() => {
+      process.env.GITHUB_SERVER_URL = 'https://enterprise.git.com'
+    })
+
+    afterEach(() => {
+      delete process.env.GITHUB_SERVER_URL
+    })
+
+    it('getDefaultBranch should use GITHUB_SERVER_URL to set the baseUrl', async () => {
+      ;(github.getOctokit as jest.Mock).mockImplementation(() => {
+        return {
+          rest: {
+            repos: {
+              get: jest.fn(() => ({data: {default_branch: 'default-branch'}}))
+            }
+          }
+        }
+      })
+
+      await githubApiHelper.getDefaultBranch('token', 'owner', 'repo')
+
+      expect(github.getOctokit).toHaveBeenCalledWith(
+        'token',
+        expect.objectContaining({
+          baseUrl: 'https://enterprise.git.com/api/v3'
+        })
+      )
+    })
+  })
+})