Add Scala 2.13.8 to plugin cross-compilation

[jackkoenig]

Needed so that 3.5.1 [plugin] release will be available for Scala 2.13.8

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

Downstream users will now be able to use Scala 2.13.8 (compiler plugin must be published for every minor version)

Backend Code Generation Impact

No impact

Desired Merge Strategy

• Squash

Release Notes

The chisel3 plugin is now published for Scala 2.13.8.

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (Bug fix: 3.4.x, [small] API extension: 3.5.x, API modification or big change: 3.6.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you mark as Please Merge?