[ruby-kit] dev-ruby/rubygems for ruby3.1 doesn't emerge

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```
Step 13/16 : RUN eval 'mkdir -p /var/cache/portage/distfiles || true' && chown portage:portage -R /var/cache/portage/ && USE="-ruby_targets_ruby27 -ruby_targets_ruby30 -ruby_targets_ruby32 -test ruby_targets_ruby31" emerge dev-ruby/rubygems -pv -t
---> Running in 9d852ed4fe07

These are the packages that would be merged, in reverse order:

Calculating dependencies ... done!
[ebuild N ] dev-ruby/rubygems-3.6.6::ruby-kit USE="-server -test" RUBY_TARGETS="ruby31 -ruby27 -ruby30 -ruby32" 12,652 KiB

Total: 1 package (1 new), Size of downloads: 12,652 KiB
Removing intermediate container 9d852ed4fe07
---> f2522635a0ee
Step 14/16 : RUN USE="-ruby_targets_ruby27 -ruby_targets_ruby30 -ruby_targets_ruby32 -test ruby_targets_ruby31" emerge -j ${JOBS} --nodeps dev-ruby/rubygems
---> Running in 074b65ee8944
>>> Verifying ebuild manifests
>>> Jobs: 0 of 1 complete, 1 running Load avg: 2.15, 2.54, 4.57
>>> Emerging (1 of 1) dev-ruby/rubygems-3.6.6::ruby-kit
>>> Jobs: 0 of 1 complete, 1 running Load avg: 2.15, 2.54, 4.57
>>> Failed to emerge dev-ruby/rubygems-3.6.6, Log file:
>>> Jobs: 0 of 1 complete, 1 running Load avg: 2.06, 2.51, 4.55
>>> '/var/tmp/portage/dev-ruby/rubygems-3.6.6/temp/build.log'
>>> Jobs: 0 of 1 complete, 1 running Load avg: 2.06, 2.51, 4.55
>>> Jobs: 0 of 1 complete, 1 running, 1 failed Load avg: 2.06, 2.51, 4.55
>>> Jobs: 0 of 1 complete, 1 failed Load avg: 2.06, 2.51, 4.55

* Package: dev-ruby/rubygems-3.6.6
* Repository: ruby-kit
* USE: amd64 elibc_glibc kernel_linux ruby_targets_ruby31 userland_GNU
* FEATURES: preserve-libs userpriv
>>> Unpacking source...
>>> Unpacking rubygems-3.6.6-25cf076.tar.gz to /var/tmp/portage/dev-ruby/rubygems-3.6.6/work/all
>>> Source unpacked in /var/tmp/portage/dev-ruby/rubygems-3.6.6/work
>>> Preparing source in /var/tmp/portage/dev-ruby/rubygems-3.6.6/work ...
* Running prepare phase for all ...
* Running prepare phase for all ...
cp: cannot stat '/var/tmp/portage/dev-ruby/rubygems-3.6.6/repo/dev-ruby/files/rubygems/gentoo-defaults-5.rb': No such file or directory
* ERROR: dev-ruby/rubygems-3.6.6::ruby-kit failed (prepare phase):
* (no error message)
*
* Call stack:
* ebuild.sh, line 93: Called src_prepare
* environment, line 1922: Called ruby-ng_src_prepare
* environment, line 1682: Called _ruby_invoke_environment 'all' '_ruby_apply_patches'
* environment, line 555: Called _ruby_apply_patches
* environment, line 379: Called all_ruby_prepare
* environment, line 606: Called die
* The specific snippet of code:
* cp "${REPODIR}/dev-ruby/files/${PN}/gentoo-defaults-5.rb" lib/rubygems/defaults/operating_system.rb || die;
*
* If you need support, post the output of `emerge --info '=dev-ruby/rubygems-3.6.6::ruby-kit'`,
* the complete build log and the output of `emerge -pqv '=dev-ruby/rubygems-3.6.6::ruby-kit'`.
* The complete build log is located at '/var/tmp/portage/dev-ruby/rubygems-3.6.6/temp/build.log'.
* The ebuild environment file is located at '/var/tmp/portage/dev-ruby/rubygems-3.6.6/temp/environment'.
* Working directory: '/var/tmp/portage/dev-ruby/rubygems-3.6.6/work/all/rubygems-3.6.6'
* S: '/var/tmp/portage/dev-ruby/rubygems-3.6.6/work/all/rubygems-3.6.6'
```

Start with the dev-ruby/rubygems-3.6.6 ebuild and its ruby-ng_src_prepare path, then inspect why /var/tmp/portage/dev-ruby/rubygems-3.6.6/repo/dev-ruby/files/rubygems/gentoo-defaults-5.rb is expected but missing. Reproduce the shown emerge command and use the build log to confirm the failure is resolved when the package completes its prepare phase.

Written by the indexing model from the issue text.