Use Pigweed boringssl mirror

Change-Id: I2fb5dd0bcbb2ed2d9e948c02d7aa8aaa9d2663a9
Reviewed-on: https://pigweed-review.googlesource.com/c/open-dice/+/34060
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Darren Krahn <dkrahn@google.com>
diff --git a/.gitmodules b/.gitmodules
index 95345a8..3576722 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,6 +1,6 @@
 [submodule "boringssl"]
 	path = third_party/boringssl/src
-	url = https://boringssl.googlesource.com/boringssl
+	url = https://pigweed.googlesource.com/third_party/boringssl/boringssl
 [submodule "mbedtls"]
 	path = third_party/mbedtls/src
 	url = https://pigweed.googlesource.com/third_party/github/ARMmbed/mbedtls.git