Fix rubygems update when non default --install-dir is configured

[deivid-rodriguez]

What was the end-user or developer problem that led to this PR?

If RubyGems is configured with a non default --install-dir, gem update --system will fail because it tries to switching to the rubygems-update installation folder in the default gem dir, and the gem is not there.

What is your fix for the problem, implemented in this PR?

Use spec.base_dir of the installed updater gem, instead of always using Gem.dir.

Fixes #5541.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[brettcurtis]

Is this problem back again?

RubyGems Environment:
  - RUBYGEMS VERSION: 3.3.8
  - RUBY VERSION: 3.1.3 (2022-11-24 patchlevel 185) [x86_64-linux]
  - INSTALLATION DIRECTORY: /usr/lib64/ruby/gems/3.1.0
  - USER INSTALLATION DIRECTORY: /root/.local/share/gem/ruby/3.1.0
  - RUBY EXECUTABLE: /usr/bin/ruby31
  - GIT EXECUTABLE: /usr/bin/git
  - EXECUTABLE DIRECTORY: /usr/bin
  - SPEC CACHE DIRECTORY: /root/.local/share/gem/specs
  - SYSTEM CONFIGURATION DIRECTORY: /etc
  - RUBYGEMS PLATFORMS:
     - ruby
     - x86_64-linux
  - GEM PATHS:
     - /usr/lib64/ruby/gems/3.1.0
     - /root/.local/share/gem/ruby/3.1.0
     - /usr/local/lib64/ruby/gems/3.1.0
  - GEM CONFIGURATION:
     - :update_sources => true
     - :verbose => true
     - :backtrace => false
     - :bulk_threshold => 1000
     - "install" => "--install-dir /usr/local/lib64/ruby/gems/3.1.0 --bindir /usr/local/bin"
     - "uninstall" => "--install-dir /usr/local/lib64/ruby/gems/3.1.0 --bindir /usr/local/bin"
     - "update" => "--install-dir /usr/local/lib64/ruby/gems/3.1.0 --bindir /usr/local/bin"
  - REMOTE SOURCES:
     - https://rubygems.org/
  - SHELL PATH:
     - /home/brett/google-cloud-sdk/bin
     - /home/brett/google-cloud-sdk/bin
     - /home/brett/bin
     - /home/brett/go/bin
     - /home/brett/.local/bin
     - /usr/local/sbin
     - /usr/local/bin
     - /usr/sbin
     - /usr/bin
     - /sbin
     - /bin
     - /opt/bin

❯ sudo gem update --system
Updating rubygems-update
Fetching rubygems-update-3.3.26.gem
Successfully installed rubygems-update-3.3.26
ERROR:  While executing gem ... (Errno::ENOENT)
    No such file or directory @ dir_chdir - /usr/lib64/ruby/gems/3.1.0/gems/rubygems-update-3.3.26

[deivid-rodriguez]

I think the explanation is that you need the initial version of RubyGems to have the fix, and the fix was released with 3.3.15 but you are upgrading from 3.3.8.