CI: Github Actions: temporarily disable BoringSSL builds

A recent update to BoringSSL broke the build again, and given that
it's not used except for QUIC development, let's temporarily disable
it until the issue is analysed and fixed.
diff --git a/.github/matrix.py b/.github/matrix.py
index 0478aa4..e3334a0 100755
--- a/.github/matrix.py
+++ b/.github/matrix.py
@@ -113,7 +113,7 @@
         "OPENSSL_VERSION=3.0.0-alpha17",
         "LIBRESSL_VERSION=2.9.2",
         "LIBRESSL_VERSION=3.3.3",
-        "BORINGSSL=yes",
+#        "BORINGSSL=yes",
     ]:
         flags = ["USE_OPENSSL=1"]
         if ssl == "BORINGSSL=yes":