[ruby-core:118964] [Ruby master Bug#20700] `warn_cr_in_shebang()` doesn't fire if there's no `\n`

["AMomchilov (Alexander Momchilov) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20700 has been reported by AMomchilov (Alexander Momchilov).

----------------------------------------
Bug #20700: `warn_cr_in_shebang()` doesn't fire if there's no `\n`
https://bugs.ruby-lang.org/issues/20700

* Author: AMomchilov (Alexander Momchilov)
* Status: Open
* ruby -v: ruby 3.3.4 (2024-07-09 revision be1089c8ec) [arm64-darwin23]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
The `warn_cr_in_shebang()` function looks for `\r\n` together, but the `\r` should emit a warning, regardless if there's also a `\n` or not.

```ruby
#!/usr/bin/ruby

require "tempfile"
require "open3"

def run_rb(ruby_src)
  puts "===== Output of #{ruby_src.inspect}:"
  Tempfile.create("script.rb") do |f|
    f.print(ruby_src)
    f.close
    
    stdout_and_stderr_str, status = Open3.capture2e("/opt/rubies/3.3.4/bin/ruby", f.path)
    raise unless status.success?
    puts stdout_and_stderr_str
  end
end

run_rb("#!ruby")     # ✅ no warning
run_rb("#!ruby\r")   # ❌ no warning!
run_rb("#!ruby\r\n") # ✅ warns
run_rb("#!ruby -Eutf-8\r\n123") # ✅ warns
```



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