build_api.py: Support tee-log via environment variable.

See go/build-log-compatibility for short doc.

BUG=chromium:1012460
TEST=manual

Change-Id: Ib79233a723060e023d10448ab2a2c89a5c992a33
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1984881
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Michael Mortensen <mmortensen@google.com>
Tested-by: Michael Mortensen <mmortensen@google.com>
diff --git a/scripts/build_api_unittest.py b/scripts/build_api_unittest.py
index dcb8b73..ece1044 100644
--- a/scripts/build_api_unittest.py
+++ b/scripts/build_api_unittest.py
@@ -46,3 +46,12 @@
                     'chromite.api.PackageService/GetTargetVersions'])
     contents = osutils.ReadFile(self.tee_log)
     self.assertIn('Teeing stdout', contents)
+
+  def testEnvTee(self):
+    """Call build_api with tee-log set, verify log contents."""
+    os.environ['BUILD_API_TEE_LOG_FILE'] = self.tee_log
+    build_api.main(['--input-json', self.input_json,
+                    '--output-json', self.output_json,
+                    'chromite.api.PackageService/GetTargetVersions'])
+    contents = osutils.ReadFile(self.tee_log)
+    self.assertIn('Teeing stdout and stderr to env path ', contents)