[DRE-maint] Bug#890751: rake FTBFS with Ruby 2.5: cannot load such file -- ubygems (LoadError)

[Adrian Bunk]

Source: rake
Version: 12.3.0-1
Severity: serious

https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/rake.html

...
┌──────────────────────────────────────────────────────────────────────────────┐
│ Run tests for ruby2.5 from debian/ruby-tests.rb                              │
└──────────────────────────────────────────────────────────────────────────────┘

RUBYLIB=/build/1st/rake-12.3.0/debian/rake/usr/lib/ruby/vendor_ruby:. GEM_PATH=debian/rake/usr/share/rubygems-integration/all:/var/lib/gems/2.5.0:/usr/lib/x86_64-linux-gnu/rubygems-integration/2.5.0:/usr/share/rubygems-integration/2.5.0:/usr/share/rubygems-integration/all ruby2.5 debian/ruby-tests.rb
/usr/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require': cannot load such file -- ubygems (LoadError)
	from /usr/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'
ERROR: Test "ruby2.5" failed. Exiting.
dh_auto_install: dh_ruby --install /build/1st/rake-12.3.0/debian/rake returned exit code 1
make: *** [debian/rules:4: binary] Error 1