Revise isGhes logic (#511)

* Revise `isGhes` logic

* ran `npm run format`

* added unit test

* tweaked unit test

* ran `npm run format`

__tests__/cache-utils.test.ts

@@ -209,3 +209,41 @@ describe('isCacheFeatureAvailable', () => {
     expect(warningSpy).toHaveBeenCalledWith(warningMessage);
   });
 });
+
+describe('isGhes', () => {
+  const pristineEnv = process.env;
+
+  beforeEach(() => {
+    jest.resetModules();
+    process.env = {...pristineEnv};
+  });
+
+  afterAll(() => {
+    process.env = pristineEnv;
+  });
+
+  it('returns false when the GITHUB_SERVER_URL environment variable is not defined', async () => {
+    delete process.env['GITHUB_SERVER_URL'];
+    expect(cacheUtils.isGhes()).toBeFalsy();
+  });
+
+  it('returns false when the GITHUB_SERVER_URL environment variable is set to github.com', async () => {
+    process.env['GITHUB_SERVER_URL'] = 'https://github.com';
+    expect(cacheUtils.isGhes()).toBeFalsy();
+  });
+
+  it('returns false when the GITHUB_SERVER_URL environment variable is set to a GitHub Enterprise Cloud-style URL', async () => {
+    process.env['GITHUB_SERVER_URL'] = 'https://contoso.ghe.com';
+    expect(cacheUtils.isGhes()).toBeFalsy();
+  });
+
+  it('returns false when the GITHUB_SERVER_URL environment variable has a .localhost suffix', async () => {
+    process.env['GITHUB_SERVER_URL'] = 'https://mock-github.localhost';
+    expect(cacheUtils.isGhes()).toBeFalsy();
+  });
+
+  it('returns true when the GITHUB_SERVER_URL environment variable is set to some other URL', async () => {
+    process.env['GITHUB_SERVER_URL'] = 'https://src.onpremise.fabrikam.com';
+    expect(cacheUtils.isGhes()).toBeTruthy();
+  });
+});