Fix the Ruby extensions build

[casperisfine]

Based on #7027

I finally managed to reproduce locally with:

docker run -v $PWD:/tmpfs/src/github/protobuf/ruby -e UID\=1000 -e GID\=1000 -e USER\=kbuilder -e GROUP\=kbuilder -e GEM_PRIVATE_KEY_PASSPHRASE -e ftp_proxy -e http_proxy -e https_proxy -e RCD_HOST_RUBY_PLATFORM\=x86_64-linux -e RCD_HOST_RUBY_VERSION\=2.4.1 -e RCD_IMAGE\=protobuftesting/ruby_rake_compiler_52bbc96c6d72ba1b25e99e19bdcfeb240438566c -w /tmpfs/src/github/protobuf/ruby --rm -i protobuftesting/ruby_rake_compiler_52bbc96c6d72ba1b25e99e19bdcfeb240438566c runas sigfw bash -c 'bundle && IN_DOCKER=true rake cross native gem RUBY_CC_VERSION\=2.7.0:2.6.0:2.5.0:2.4.0:2.3.0]'

Installing bundler just before bundle install like this is not super elegant, but given that I don't have a proper feedback loop, it seems like the most straightforward fix.

@qnighy @haberman

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[casperisfine]

@googlebot I consent.

[acozzette]

@casperisfine I think the CLA bot is having trouble validating that all CLAs are signed because of the commits by @byroot and @qnighy. If they leave comments saying @googlebot I consent then that should solve the problem.

[byroot]

@googlebot I consent.

[qnighy]

@googlebot I consent.

[googlebot]

CLAs look good, thanks!

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[haberman]

Kokoro build appears broken still?

[acozzette]

I'm going to close this because I assume it's no longer needed since #7027 was merged.