allow overriding system trust store location via $NIX_SSL_CERT_FILE

--- a/lib/system/certs.c
+++ b/lib/system/certs.c
@@ -381,6 +381,10 @@ int gnutls_x509_trust_list_add_system_trust(gnutls_x509_trust_list_t list,
 					    unsigned int tl_flags,
 					    unsigned int tl_vflags)
 {
-	return add_system_trust(list, tl_flags | GNUTLS_TL_NO_DUPLICATES,
-				tl_vflags);
+	tl_flags = tl_flags|GNUTLS_TL_NO_DUPLICATES;
+	const char *file = secure_getenv("NIX_SSL_CERT_FILE");
+	return file
+		? gnutls_x509_trust_list_add_trust_file(
+			list, file, NULL/*CRL*/, GNUTLS_X509_FMT_PEM, tl_flags, tl_vflags)
+		: add_system_trust(list, tl_flags, tl_vflags);
 }