Temporary PR deployments should have the source header populated with the PR number

[petertonysmith94]

Summary

For temporary PR deployments, we should be able to identify via the Source header which version is being used. We could either:

1. Read the version installed on the user's disk
2. Add some automation to update the getBuiltinVersions return for temporary releases.

Originally posted by @nedsalk in #3282 (comment)

Follow up from: #3280

[petertonysmith94]

Questions:

With option 1: how do we then add these to the header?
With option 2: this will cause tests to fail.

cc @nedsalk

[arboleya]

This is low priority, especially given our moment, and can be circumvented by manually editing the version file.

[Dhaiwat10]

Unassigning myself for now then