Corrects shebang of bxpb-protoc-plugin.

Refs #1.

CI appears to be failing, due to a missing `packages/bxpb-protoc-plugin/bin/bxpb-protoc-plugin` file. This is weird to me as the file is directly checked in to source.

https://github.com/dgp1130/bxpb/runs/702931728

The symlink appears to work as expected after passing through a wrapper shell script from lerna/lerna#2059, and that script works correctly when used locally.

While doing some investigation, I noticed an NPM best practice of using `#!/usr/bin/env node` for the shebang line. I'm hoping GitHub actions just has the Node binary in a different location and printed a very bad error message. Only one way to find out!

packages/bxpb-protoc-plugin/src/bxpb-protoc-plugin.ts

@@ -1,4 +1,4 @@
-#!/usr/bin/node
+#!/usr/bin/env node
 
 import { execute } from './plugin';