certs: add 'USERTrust ECC Certification Authority'

Add ROOT certification authority for github.com
(USERTrust ECC Certification Authority)

Also, running 'make' when you don't have support for ipv6
stops at 'ipv6.showipv6.de' which only has AAAA record.

To allow verify all IPv4 sites/certs before failing for IPv6,
Makefile now has two domain lists; the original one (DOMAINS)
and the IPv6-only (DOMAINSIPV6).

With these changes, the error occurs after validating all
IPv4 compatible sites/certs.

certs/Makefile

@@ -12,9 +12,9 @@ DOMAINS = \
 	dns.quad9.net/DigiCert-Global-Root-G3 \
 	feodotracker.abuse.ch/GlobalSign \
 	git.eworm.de/ISRG-Root-X2 \
+	github.com/USERTrust-ECC-Certification-Authority \
 	ipv4.showipv6.de/ISRG-Root-X1 \
 	ipv4.tunnelbroker.net/Starfield-Root-Certificate-Authority-G2 \
-	ipv6.showipv6.de/ISRG-Root-X1 \
 	lists.blocklist.de/Certum-Trusted-Network-CA \
 	matrix.org/GTS-Root-R4 \
 	mkcert.org/ISRG-Root-X1 \
@@ -24,9 +24,12 @@ DOMAINS = \
 	www.dshield.org/ISRG-Root-X1 \
 	www.spamhaus.org/GTS-Root-R4
 
-.PHONY: $(DOMAINS)
+DOMAINSIPV6 = \
+	ipv6.showipv6.de/ISRG-Root-X1
 
-all: $(DOMAINS)
+.PHONY: $(DOMAINS) $(DOMAINSIPV6)
 
-$(DOMAINS):
+all: $(DOMAINS) $(DOMAINSIPV6)
+
+$(DOMAINS) $(DOMAINSIPV6) :
 	curl --output /dev/null --silent --connect-timeout 5 --cacert $(notdir $@).pem https://$(dir $@)