Support installing a specific buck2 release

[joshlafleur-zip]

Reason for Change

The current action implementation allows for facebook to release a new latest software, and subsequently breaking downstream action consumers based on buck2 version incompatibility (prelude included).

Description of Change

• Support a release variable which defaults to latest
• Document the action input

Test Plan

• Run action with release specified, ensure the right build is installed ✅
• Ensure the buck2 build succeeds (this was previously passing in our repo) ✅

[joshlafleur-zip]

Hey @dtolnay, we use your action in a student project for CI builds and following facebook's release of a new latest today we experienced a build breakage due to buck2 incompatibility. We mitigated this internally by forking and applying this patch, however I am bubbling this upstream as well in case others have a similar use case or want to support pinning the buck2 release. Happy to chat if there is any desire to change this, but it feels straightforward enough here?