Read the server url from the environment variable.

Instead of having the urls hardcoded, read them from the environment.
I opted to read from the environment variable instead of the github context
because it would be easier to test.

packages/attest/__tests__/endpoints.test.ts

@@ -0,0 +1,41 @@
+import {signingEndpoints} from '../src/endpoints'
+
+describe('signingEndpoints', () => {
+  const originalEnv = process.env
+
+  afterEach(() => {
+    process.env = originalEnv
+  })
+
+  describe('when using github.com', () => {
+    beforeEach(async () => {
+      process.env = {
+        ...originalEnv,
+        GITHUB_SERVER_URL: 'https://github.com'
+      }
+    })
+
+    it('returns expected endpoints', async () => {
+      const endpoints = signingEndpoints('github')
+
+      expect(endpoints.fulcioURL).toEqual('https://fulcio.githubapp.com')
+      expect(endpoints.tsaServerURL).toEqual('https://timestamp.githubapp.com')
+    })
+  })
+
+  describe('when using custom domain', () => {
+    beforeEach(async () => {
+      process.env = {
+        ...originalEnv,
+        GITHUB_SERVER_URL: 'https://foo.bar.com'
+      }
+    })
+
+    it('returns a expected endpoints', async () => {
+      const endpoints = signingEndpoints('github')
+
+      expect(endpoints.fulcioURL).toEqual('https://fulcio.foo.bar.com')
+      expect(endpoints.tsaServerURL).toEqual('https://timestamp.foo.bar.com')
+    })
+  })
+})