REGTESTS: set_ssl_server_cert.vtc: set as broken

It looks like this test is broken with a low nbthread value (1 for
example). Disable this test in the CI until the problem is solved.
diff --git a/reg-tests/ssl/set_ssl_server_cert.vtc b/reg-tests/ssl/set_ssl_server_cert.vtc
index fa3152b..0a58c1b 100644
--- a/reg-tests/ssl/set_ssl_server_cert.vtc
+++ b/reg-tests/ssl/set_ssl_server_cert.vtc
@@ -1,4 +1,4 @@
-#REGTEST_TYPE=devel
+#REGTEST_TYPE=broken
 
 # This reg-test uses the "set ssl cert" command to update a backend certificate over the CLI.
 # It requires socat to upload the certificate