ci: bump ghasum from v0.6.1 to v0.6.3

[ericcornelissen]

Upgrade the ghasum binary and the local ghasum action.

For more details see chains-project/ghasum@v0.6.1...v0.6.3 or just ask 🙂

To clarify, the changes to the local action are as follows (in order):

• Bump the ghasum version and corresponding checksum.
• Support a "sha256:" prefix on the checksums.
• If a problem is detected, removes all actions from the runner so that subsequent steps as well as post scripts cannot execute.