Add platforms input to build workflow

[Ronster2018]

Steps taken to test the fix

• Deleted the previous packages located in actions_playground
• Confirmed that the platform variable was passed in the job
• Ensured that the right platform was being passed to the commands that build the image.
• Pulled the image via docker pull ghcr.io/hathitrust/actions_playground:unstable and found "Architecture": "amd64" after inspecting the image.

Since these changes worked in the playground repository, they should work here too.

[aelkiss]

Makes sense to me. Looking across our other repos, is this generally an issue (needing to pass through platforms), or was it only missing for otis?

[coveralls]

coverage: 98.225%. remained the same
when pulling 7189d0c on ETT-696
into 23eeb55 on main.

[Ronster2018]

Makes sense to me. Looking across our other repos, is this generally an issue (needing to pass through platforms), or was it only missing for otis?

It's a general issue and I will be doing this across all repos that use the build action.