Ignore base64 gem during update_gems workflow

[Sharpie]

Short description

Prior to this commit, the update_gems workflow would add a pkg.build_requires 'base64' statement when updating gem components. This would cause the update to fail tests as apt install rubygems-base64 or dnf install rubygems-base64 doesn't work on many platforms.

This commit restores base64 to an "ignored" status as it is, depending on Ruby version, a default or bundled gem that is always present in the installation in some form. There may be issues surrounding base64, but it seems better for a human to sort those out manually than for automation to repeatedly make the wrong decision every day at midnight.

This partially reverts commit 2dcd0db.

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• read and accepted the AI Policy document and added Generated-by or Assisted-by annotations to each of my commits created with the help of an AI agent
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[Sharpie]

Also, given pkg.build_requires is leading to apt or dnf being run instead of something happening with components/rubygem-base64.rb, I suspect these pkg.build_requires statements may not be doing what we expect them to be doing.

But, that is a dragon to slay another day.