CI: ssl: enable parallel builds for OpenSSL on Linux

Running the "make all" phase on my machine with -j$(nproc) shrinks the
build time from 4'52 to 1'28. It will not be that big of a change in
the CI since it looks like two CPUs are exposed, but it should still
remain a net win. Let's enable it. The install phase obviously remains
sequential however.

(cherry picked from commit c6f1498405c9d118aa7df67815d6111738c16e28)
Signed-off-by: William Lallemand <wlallemand@haproxy.org>
diff --git a/scripts/build-ssl.sh b/scripts/build-ssl.sh
index 9a6a2b2..b36593e 100755
--- a/scripts/build-ssl.sh
+++ b/scripts/build-ssl.sh
@@ -21,7 +21,8 @@
     (
         cd "openssl-${OPENSSL_VERSION}/"
         ./config shared --prefix="${HOME}/opt" --openssldir="${HOME}/opt" -DPURIFY
-        make all install_sw
+        make -j$(nproc) all
+        make install_sw
     )
 }