Bump to Scala 2.12.13

[jackkoenig]

Contributor Checklist

• [NA] Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• [NA] Did you add appropriate documentation in docs/src?
• Did you state the API impact?
• Did you specify the code generation impact?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• new feature/API

API Impact

This allows users of the Chisel plugin to use Scala 2.12.13 (plugins require publishing for each minor version)

Backend Code Generation Impact

No impact

Desired Merge Strategy

• Squash

Release Notes

Bump to Scala 2.12.13, publish chisel3-plugin for 2.12.13.

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (3.2.x, 3.3.x, 3.4.0, 3.5.0) ?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you mark as Please Merge?

[jackkoenig]

As this modifies the testing workflow, it will require manual backporting.