[File blob] Use label variant of GlButton to copy commit SHA

Problem to solve

This issue represents a finding from our work to audit features belonging to groupsource code. See the analysis in Figma for more information. See the overview video for guidance on how to review this analysis.

Screenshots

Proposal

• Address the following finding on the File blob page:
  • Use label variant of GlButton to copy commit SHA
  • There is an existing Pajamas solution to address this
• Finding yellow #3