Performance workflow cannot find artifact in branches on `workflow_dispatch`

[desrosj]

The performance workflow has been failing for branches when triggered using workflow_dispatch since the changes related to #61213.

This seems to be happening due to a lack of a github.event.before property in the event payload for workflow_dispatch, which results in TARGET_SHA being set to an empty value.

This attempts to use native git commands to find the preceding commit SHA when one is not provided through GitHub Actions contexts.

Trac ticket: https://core.trac.wordpress.org/ticket/61699

[desrosj]

In 58767:

Build/Test Tools: Ensure TARGET_SHA is set for older branches.

The workflow_dispatch event payload does not contain the before property containing the previous commit’s SHA value, which causes scheduled runs of the performance testing workflow in older branches to fail.

This adds a step specifically for this event type to use native Git commands to retrieve the required SHA value instead.

Props joemcgill.
Fixes #61699.

Merged in https://core.trac.wordpress.org/changeset/58767. Thank you!