Fixes #357 Use a read stream for artifacts.

This seems to be safe since octokit mentions ReadableStreams in their fetcher wrapper & tests: https://github.com/octokit/request.js/blob/main/src/fetch-wrapper.ts#L44
(and also it seems to work when I test it).

dist/index.js

@@ -205,7 +205,7 @@ class Artifact {
         return (0, fs_1.statSync)(this.path).size;
     }
     readFile() {
-        return (0, fs_1.readFileSync)(this.path);
+        return (0, fs_1.createReadStream)(this.path);
     }
 }
 exports.Artifact = Artifact;