Use fetch.log's absolute path when symlinking

Some versions of assemble_cvd don't set the images_dir property in the
config file. Without that, fetch.log is sometimes found at ./fetch.log,
but the symbolinc link is created elsewhere, so the absolute path must
be used.

Bug: b/460022351

Author: jemoreira

base/cvd/cuttlefish/host/commands/start/main.cc

@@ -194,7 +194,8 @@ Result<void> LinkLogs2InstanceDir(
         fmt::format("{}/{}", android::base::Dirname(images_dir), "fetch.log");
   }
   if (FileExists(fetch_log)) {
-    CF_EXPECT(Symlink(fetch_log, instance.PerInstanceLogPath("fetch.log")));
+    CF_EXPECT(Symlink(AbsolutePath(fetch_log),
+                      instance.PerInstanceLogPath("fetch.log")));
   }
 
   return {};