CI: SSL: use proper version generating when "latest" semantic is used

both "OPENSSL_VERSION=latest" and "LIBRESSL_VERSION=latest" processing
introduced errors when build-ssl.sh script was invoked. that error
in turn led to skipping custom openssl build and haproxy was linked against
stock openssl, i.e. openssl-1.1.1
diff --git a/.github/matrix.py b/.github/matrix.py
index 6282767..88dbaa8 100755
--- a/.github/matrix.py
+++ b/.github/matrix.py
@@ -42,7 +42,7 @@
         if "openssl-" in name:
             if name > latest_tag:
                latest_tag = name
-    return "OPENSSL={}".format(latest_tag[8:])
+    return "OPENSSL_VERSION={}".format(latest_tag[8:])
 
 def determine_latest_libressl(ssl):
     libressl_download_list = urllib.request.urlopen("http://ftp.openbsd.org/pub/OpenBSD/LibreSSL/")
@@ -50,7 +50,7 @@
         decoded_line = line.decode("utf-8")
         if "libressl-" in decoded_line and ".tar.gz.asc" in decoded_line:
              l = re.split("libressl-|.tar.gz.asc", decoded_line)[1]
-    return "LIBRESSL={}".format(l)
+    return "LIBRESSL_VERSION={}".format(l)
 
 def clean_compression(compression):
     return compression.replace("USE_", "").lower()