workflows: switch to SHA rather than ref

[patrick-stephens]

Signed-off-by: Patrick Stephens [email protected]

Fixes #4999 by using the SHA as the tag name.
This allows us to identify the specific image used in each integration test run rather than a floating latest one with its inherent race conditions between build & provision+test.

Unfortunately as this is pull_request_target it will run with the base workflow.

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