Makefile: Fix the SSL certs configuration of Guix container.

* Makefile (portable-release-tarball): Do expose SSL_CERT_FILE
directly; expose SSL_CERT_DIR as /etc/ssl/certs instead, which is
where GnuTLS (hence wget) looks.

Change-Id: If827b33134a0846317efc0034ed27bd122334e23

Makefile

@@ -97,7 +97,7 @@ portable-release-tarball:
 	guix environment --container --network \
           --preserve=TARBALLS $(guix-share-tarball-arg) \
           --expose=/usr/bin/env \
-          --expose=$$SSL_CERT_FILE \
+          --expose=$$SSL_CERT_DIR=/etc/ssl/certs \
           --manifest=$(CURDIR)/guix/minimal-manifest.scm \
           -- $(MAKE) release-tarball