deploy job is failing on release process

deploy job is failing on release process.

https://gitlab.com/gitlab-org/labkit-ruby/-/jobs/1201744787 (part of https://gitlab.com/gitlab-org/labkit-ruby/-/pipelines/290346249)

$ tools/deploy-rubygem.sh
+ rm -rf pkg/
+ mkdir -p pkg/
+ bundle install
Fetching gem metadata from https://rubygems.org/.......
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Bundler could not find compatible versions for gem "ruby":
  In Gemfile:
    ruby
    gitlab-labkit was resolved to 0.17.0, which depends on
      ruby (>= 2.6.0)
Could not find gem 'ruby (>= 2.6.0)', which is required by gem 'gitlab-labkit',
in any of the relevant sources:
  the local ruby installation
[...]
ERROR: Job failed: exit code 1

Since we do not specify the base image for jobs, the default image ruby:2.5 was used.