CI: Use RUBYOPT: "--enable-frozen-string-literal" to find any incompatibilities

[olleolleolle]

This PR makes a try-out with the "frozen string literals" setting.

If it works, then great, if not, we learned something. Update: We learned something.

Now, the only failures that are left come from the multipart-post gem, and its next release will fix those, too.

This PR fixes the parts in octokit.rb which used strings in incompatible ways.

---

Changes

• build_error_message could use a mutable string 4dcea1c, 9f9c7a0
• CI: use an updated version (still 2.x) of RubyGems for Ruby 2.5

Learnings

• ✅ Ruby 2.5's old-but-shippped rubygems version had an issue - Ruby 2.5 with rubygems 2.7.6.3: bundler_version_finder.rb:89:in `untaint': can't modify frozen String (FrozenError) rubygems/rubygems#4837 - so we could just update away from that problem for 2.5.
• ✅ the multipart-post gem had a frozen string literal issue, now fixed parts.rb: Use a mutable string socketry/multipart-post#70

[olleolleolle]

I learned 📖 that one file in RubyGems is modifying a string in-place: with Ruby 2.5, this became a problem.

https://github.com/rubygems/rubygems/blob/master/lib/rubygems/bundler_version_finder.rb#L87

Output from test:

/opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/bundler_version_finder.rb:89:in `untaint': can't modify frozen String (FrozenError)
	from /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/bundler_version_finder.rb:89:in `lockfile_contents'
	from /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/bundler_version_finder.rb:65:in `lockfile_version'
	from /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/bundler_version_finder.rb:19:in `bundler_version_with_reason'
	from /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/bundler_version_finder.rb:4:in `bundler_version'
	from /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/bundler_version_finder.rb:42:in `filter!'
	from /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/dependency.rb:283:in `matching_specs'
	from /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems.rb:273:in `find_spec_for_exe'
	from /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems.rb:303:in `activate_bin_path'
	from /opt/hostedtoolcache/Ruby/2.5.9/x64/bin/bundle:23:in `<main>'

Update: this has been fixed in later releases of RubyGems.

[timrogers]

Sadly, it looks like the fix on multipart-post still hasn't been released into a version, so this is stuck until then. I'll try creating an issue over there.

[timrogers]

Changed merged in #1426.