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.

scripts/build-ssl.sh

@@ -21,7 +21,8 @@ build_openssl_linux () {
     (
         cd "openssl-${OPENSSL_VERSION}/"
         ./config shared --prefix="${HOME}/opt" --openssldir="${HOME}/opt" -DPURIFY
-        make all install_sw
+        make -j$(nproc) all
+        make install_sw
     )
 }