Stub GITHUB_WORKSPACE in test

src/util.test.ts

@@ -536,6 +536,7 @@ test("getCgroupCpuCountFromCpus returns undefined if the CPU file exists but is
 });
 
 test("checkDiskUsage succeeds and produces positive numbers", async (t) => {
+  process.env["GITHUB_WORKSPACE"] = os.tmpdir();
   const diskUsage = await util.checkDiskUsage(getRunnerLogger(true));
   t.true(
     diskUsage?.numAvailableBytes !== undefined &&