chore: lint Ruby files with Rubocop (#2435)

* fix: set up Ruby before linting in Github Actions
* chore: separate linting from fixing

Before we commit we want to fix as much as possible, but when we're checking a
pull request we're not going to commit anything so we want to know about all
the problems, including the fixable ones.

Gemfile.lock

@@ -23,6 +23,7 @@ GEM
       httpclient (~> 2.8, >= 2.8.3)
       json (>= 1.5.1)
     artifactory (3.0.17)
+    ast (2.4.2)
     atomos (0.1.3)
     aws-eventstream (1.3.0)
     aws-partitions (1.996.0)
@@ -216,6 +217,7 @@ GEM
     json (2.7.5)
     jwt (2.9.3)
       base64
+    language_server-protocol (3.17.0.3)
     mini_magick (4.13.2)
     mini_mime (1.1.5)
     mini_portile2 (2.8.5)
@@ -233,10 +235,16 @@ GEM
       racc (~> 1.4)
     optparse (0.5.0)
     os (1.1.4)
+    parallel (1.26.3)
+    parser (3.3.6.0)
+      ast (~> 2.4.1)
+      racc
     plist (3.7.1)
     public_suffix (4.0.7)
     racc (1.7.3)
+    rainbow (3.1.1)
     rake (13.2.1)
+    regexp_parser (2.9.2)
     representable (3.2.0)
       declarative (< 0.1.0)
       trailblazer-option (>= 0.1.1, < 0.2.0)
@@ -244,7 +252,20 @@ GEM
     retriable (3.1.2)
     rexml (3.3.9)
     rouge (2.0.7)
+    rubocop (1.68.0)
+      json (~> 2.3)
+      language_server-protocol (>= 3.17.0)
+      parallel (~> 1.10)
+      parser (>= 3.3.0.2)
+      rainbow (>= 2.2.2, < 4.0)
+      regexp_parser (>= 2.4, < 3.0)
+      rubocop-ast (>= 1.32.2, < 2.0)
+      ruby-progressbar (~> 1.7)
+      unicode-display_width (>= 2.4.0, < 3.0)
+    rubocop-ast (1.36.1)
+      parser (>= 3.3.1.0)
     ruby-macho (2.5.1)
+    ruby-progressbar (1.13.0)
     ruby2_keywords (0.0.5)
     rubyzip (2.3.2)
     security (0.1.5)
@@ -292,6 +313,7 @@ DEPENDENCIES
   fastlane
   fastlane-sirp!
   nokogiri
+  rubocop
 
 RUBY VERSION
    ruby 2.7.5p203