Fix detection of compiler_wd in tool/update-deps

[eightbitraptor]

This fixes the large list of warnings at the top of the Check Dependencies CI run that look like this:

warning: file not found: /home/runner/work/ruby/ruby//
warning: file not found: /home/runner/work/ruby/ruby//
warning: file not found: /home/runner/work/ruby/ruby//
warning: file not found: /home/runner/work/ruby/ruby//
warning: file not found: /home/runner/work/ruby/ruby//
warning: file not found: /home/runner/work/ruby/ruby//
warning: file not found: /home/runner/work/ruby/ruby//
warning: file not found: /home/runner/work/ruby/ruby//
warning: file not found: /home/runner/work/ruby/ruby//
warning: file not found: /home/runner/work/ruby/ruby//
warning: file not found: /home/runner/work/ruby/ruby//
warning: file not found: /home/runner/work/ruby/ruby//
warning: file not found: /home/runner/work/ruby/ruby//
warning: file not found: /home/runner/work/ruby/ruby//
warning: file not found: /home/runner/work/ruby/ruby//

These warnings are being caused because gcc always outputs

# 1 "/home/runner/work/ruby/ruby//"

as the second line of the *.i file. There is already code in update-deps with the intention of removing this line from the dependencies array, however it is not working as intended

This is because when it loops through all the lines in the *.i files, it sets dep each time, and sets compiler_wd ||= dep. The use of ||= here means that the compiler_wd is always set to the first line in the file, rather than the second. Therefore the regex used to detect and remove the // and set the compile_wd correctly is never matching.