libcurl: honor $SSL_CERT_FILE (fixed)

The previous attempt to patch libcurl used

getenv("CURL_CA_BUNDLE") || getenv("SSL_CERT_FILE")

to get the second environment variable if the first is unset.
Unfortunately, this broke libcurl because the (||) operator is C returns
only 0 or 1, so it is inappropriate for pointer comparisons! Now we use

getenv("CURL_CA_BUNDLE") ? getenv("CURL_CA_BUNDLE") : getenv("SSL_CERT_FILE")

instead. This has one downside: it always calls getenv twice! But,
that's a small price to pay for actually being correct.