CI: SSL: temporarily stick to LibreSSL=3.5.3

recently released 3.6.0 introduced some regression which must be
resolved first, let us use 3.5.3 notation instead of "latest"
diff --git a/.github/matrix.py b/.github/matrix.py
index 88dbaa8..19ae87d 100755
--- a/.github/matrix.py
+++ b/.github/matrix.py
@@ -128,7 +128,7 @@
         "stock",
         "OPENSSL_VERSION=1.0.2u",
         "OPENSSL_VERSION=latest",
-        "LIBRESSL_VERSION=latest",
+        "LIBRESSL_VERSION=3.5.3",
         "QUICTLS=yes",
 #        "BORINGSSL=yes",
     ]: