Run script in GITHUB_WORKSPACE directory

Change to the GITHUB_WORKSPACE directory before running the script so
that callers do not have to exlicitly use the GITHUB_WORKSPACE
environment variable when requiring script files.

Workflow run steps are run within the GITHUB_WORKSPACE so this seems
like the expected default.

__test__/async-function.test.ts

@@ -25,4 +25,48 @@ describe('callAsyncFunction', () => {
   test('can access console', async () => {
     await callAsyncFunction({} as any, 'console')
   })
+
+  describe('change directory', () => {
+    const MOCK_CWD = '/path/to/action'
+    const MOCK_GITHUB_WORKSPACE = '/path/to/code'
+    const PREV_GITHUB_WORKSPACE = process.env.GITHUB_WORKSPACE
+
+    let chdir: jest.SpyInstance
+
+    beforeEach(() => {
+      jest.resetModules()
+      jest.clearAllMocks()
+
+      let cwd = MOCK_CWD
+
+      chdir = jest.spyOn(process, 'chdir').mockImplementation(directory => {
+        cwd = directory
+      })
+      jest.spyOn(process, 'cwd').mockImplementation(() => {
+        return cwd
+      })
+    })
+
+    afterAll(() => {
+      process.env.GITHUB_WORKSPACE = PREV_GITHUB_WORKSPACE
+    })
+
+    test('changes to GITHUB_WORKSPACE if environment variable is set', async () => {
+      process.env.GITHUB_WORKSPACE = MOCK_GITHUB_WORKSPACE
+
+      await callAsyncFunction({} as any, 'process')
+
+      expect(chdir.mock.calls.length).toBe(2)
+      expect(chdir.mock.calls[0][0]).toBe(MOCK_GITHUB_WORKSPACE)
+      expect(chdir.mock.calls[1][0]).toBe(MOCK_CWD)
+    })
+
+    test('does not change directory if GITHUB_WORKSPACE is not set', async () => {
+      delete process.env.GITHUB_WORKSPACE
+
+      await callAsyncFunction({} as any, 'process')
+
+      expect(chdir.mock.calls.length).toBe(0)
+    })
+  })
 })