fix: securetransport doesn't work as expected when curl is built with multiple ssl backends

[notjaywu]

This PR is to fix the issue#12663

Given curl is built with multiple SSL backends, CAfile is set incorrectly when SecureTransport is the selected TLS backend.

[jay]

Let's hear from another dev on mac before we do this. Also we have code in tool_operate that exempts Schannel from SSL environment variables. Would it be a good idea to do the same change there?

curl/src/tool_operate.c

Lines 2590 to 2645 in 7161cb1

	/* Set the CA cert locations specified in the environment. For Windows if
	* no environment-specified filename is found then check for CA bundle
	* default filename curl-ca-bundle.crt in the user's PATH.
	*
	* If Schannel is the selected SSL backend then these locations are
	* ignored. We allow setting CA location for schannel only when explicitly
	* specified by the user via CURLOPT_CAINFO / --cacert.
	*/
	if(tls_backend_info->backend != CURLSSLBACKEND_SCHANNEL) {
	char *env;
	env = curlx_getenv("CURL_CA_BUNDLE");
	if(env) {
	config->cacert = strdup(env);
	if(!config->cacert) {
	curl_free(env);
	curl_easy_cleanup(curltls);
	errorf(global, "out of memory");
	return CURLE_OUT_OF_MEMORY;
	}
	}
	else {
	env = curlx_getenv("SSL_CERT_DIR");
	if(env) {
	config->capath = strdup(env);
	if(!config->capath) {
	curl_free(env);
	curl_easy_cleanup(curltls);
	errorf(global, "out of memory");
	return CURLE_OUT_OF_MEMORY;
	}
	curl_free(env);
	capath_from_env = true;
	}
	env = curlx_getenv("SSL_CERT_FILE");
	if(env) {
	config->cacert = strdup(env);
	if(!config->cacert) {
	curl_free(env);
	if(capath_from_env)
	free(config->capath);
	curl_easy_cleanup(curltls);
	errorf(global, "out of memory");
	return CURLE_OUT_OF_MEMORY;
	}
	}
	}
	if(env)
	curl_free(env);
	#ifdef _WIN32
	else {
	result = FindWin32CACert(config, tls_backend_info->backend,
	TEXT("curl-ca-bundle.crt"));
	}
	#endif
	}

@nickzman

[bagder]

Let's hear from another dev on mac before we do this

I suspect @notjaywu is about the first person ever to use multissl with Secure Transport...