Fix extra newlines in output

Closes #32
See actions/toolkit#777

src/actions.js

@@ -0,0 +1,6 @@
+import { issueCommand } from "@actions/core/lib/command";
+
+// FIXME: See https://github.com/actions/toolkit/issues/777
+export function setOutput(name, value) {
+  issueCommand("set-output", { name }, value);
+}

src/main.js

@@ -5,10 +5,10 @@ import {
   info,
   notice,
   setFailed,
-  setOutput,
   warning,
 } from "@actions/core";
 import { context, getOctokit } from "@actions/github";
+import { setOutput } from "./actions.js";
 import { modifyReleaseAssets } from "./asset.js";
 import { renderReleaseBody } from "./body.js";
 import { readConfig } from "./config/reading.js";