utils/fetch: support options.agent being a boolean.

node-fetch/node-fetch#1502 changed the type of
options.agent to allow a boolean, which caused issues as we tried to call
it.  Flip the condition around and check for function instead, so that this
compiles.

Signed-off-by: Mark Yen <mark.yen@suse.com>

src/utils/fetch.ts

@@ -113,10 +113,10 @@ export default async function fetch(url: string, options?: RequestInit) {
         let agent: http.Agent;
 
         if (options?.agent) {
-          if (options.agent instanceof http.Agent) {
-            agent = options.agent;
-          } else {
+          if (typeof options.agent === 'function') {
             agent = options.agent(parsedURL);
+          } else {
+            agent = options.agent;
           }
         } else {
           agent = isSecure ? https.globalAgent : http.globalAgent;