retry_util: Force curl to use HTTP 1.1

This CL forces curl to use HTTP 1.1 due to intermittent error: "Error in
the HTTP2 framing layer".

BUG=chromium:1141133
TEST=./run_pytest ./lib/retry_util_unittest.py
TEST=./bin/cros_sdk --create --nouse-image

Change-Id: Icd6b57702a0bbc83845f4552d37d07ac7536f08c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2491264
Commit-Queue: Conor McNamara <ctmcnamara@chromium.org>
Tested-by: Conor McNamara <ctmcnamara@chromium.org>
Reviewed-by: Andrey Pronin <apronin@chromium.org>
Reviewed-by: Stephane Belmon <sbelmon@google.com>
(cherry picked from commit 4a0fcb57c0e871feaeee104dc8a152e58b10feb3)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2733959
Tested-by: Kangheui Won <khwon@chromium.org>
Auto-Submit: Kangheui Won <khwon@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Commit-Queue: Sam McNally <sammc@chromium.org>
diff --git a/lib/retry_util.py b/lib/retry_util.py
index 1a976bb..9900567 100644
--- a/lib/retry_util.py
+++ b/lib/retry_util.py
@@ -383,7 +383,7 @@
   Raises:
     DownloadError: Whenever curl fails for any reason.
   """
-  cmd = ['curl'] + curl_args
+  cmd = ['curl', '--http1.1'] + curl_args
 
   # These values were discerned via scraping the curl manpage; they're all
   # retry related (dns failed, timeout occurred, etc, see  the manpage for