subprocess worker needs default for options

[warner]

We found the subprocess-worker tests (which were disabled) were breaking because startSubprocessWorker is called without arguments, but it assumes it will receive an options object. The simplest fix is to give it a default:

diff --git a/packages/SwingSet/src/spawnSubprocessWorker.js b/packages/SwingSet/src/spawnSubprocessWorker.js
index 9bc3e0ae..6d56558e 100644
--- a/packages/SwingSet/src/spawnSubprocessWorker.js
+++ b/packages/SwingSet/src/spawnSubprocessWorker.js
@@ -20,7 +20,7 @@ const supercode = require.resolve(
 // always be Node.
 const stdio = harden(['inherit', 'inherit', 'inherit', 'pipe', 'pipe']);

-export function startSubprocessWorker(options) {
+export function startSubprocessWorker(options = {}) {
   const execPath = options.execPath || process.execPath;
   const args = options.args || ['-r', 'esm', supercode];
   const proc = spawn(execPath, args, { stdio });

however we should look at the two places from which that is called and build a more uniform signature, maybe startSubprocessWorker(execPath, options = {}).