Split linting and specs

Run them as separate steps so we see which one actually fails. This also
makes Rakefile non-executable. Unsure why it was to start with, but it
was making Truffle Ruby freak out.

.github/workflows/main.yml

@@ -32,8 +32,11 @@ jobs:
       - name: Install dependencies
         run: bundle install
 
+      - name: Lint with Rubocop
+        run: bundle exec rubocop
+
       - name: Run tests
-        run: bundle exec rake
+        run: bundle exec rspec
 
       - name: Build gem
         run: bundle exec rake build

Rakefile

@@ -1,4 +1,3 @@
-#!/usr/bin/env rake
 # frozen_string_literal: true
 
 require 'bundler/gem_tasks'