Add Rubocop CI (#7083)

* Rename IgnoredPatterns to AllowedPatterns.

* Exclude docopt.rb (not our code) from Rubocop

* Disable Style/RedundantReturn

* Disable Style/MutableConstant

* Disable Style/NumericLiterals

* Set Layout/IndentationStyle to spaces

* Temporarily disable various cops.

* Add Rubocop CI via Octocop

* Lint tree with rubocop -A -c .rubocop.yml

Co-authored-by: Satadru Pramanik <satadru@gmail.com>

packages/musl_brotli.rb

@@ -36,7 +36,7 @@ class Musl_brotli < Package
       -DWITH_STATIC_LIB=ON \
         ../ -G Ninja"
     end
-    system "PATH=#{CREW_MUSL_PREFIX}/bin:#{CREW_MUSL_PREFIX}/#{ARCH}-linux-musl#{@abi}/bin:#{ENV['PATH']} \
+    system "PATH=#{CREW_MUSL_PREFIX}/bin:#{CREW_MUSL_PREFIX}/#{ARCH}-linux-musl#{@abi}/bin:#{ENV.fetch('PATH', nil)} \
       samu -C builddir -j#{CREW_NPROC}"
   end