Add ruby 2.6 gem build for linux

[TeBoring]

No description provided.

[TeBoring]

Failed tests seem to be caused by the new mac image from kokoro.

[TeBoring]

This change modifies the script to build ruby gems, which should not be related to all these failing tests.

[Envek]

Can anyone release a new version to the RubyGems? Thanks in advance.

[oggy]

Pushing a new gem would be especially useful because one can't easily add this as a git dependency using bundler, unlike most other gems.

[Envek]

@oggy this works for me:

# Gemfile
gem "google-protobuf", git: "https://github.com/protocolbuffers/protobuf"

Isn't it works for you?

[aadityataparia]

@Envek this doesn't work,

[oggy]

@Envek No. It bundles ok, but because there are ruby files that need to be generated before the gem is built, you can't, e.g., require 'google/protobuf/struct_pb'.

[Envek]

@oggy, @aadityataparia, @romansklenar, then try to set BUNDLE_FORCE_RUBY_PLATFORM environment variable to 1 to make Bundler ignore precompiled binaries and thus build them from scratch.

See #4460 (comment) for reference.

[romansklenar]

@Envek I tried it but unfortunately, it didn't help.