fix: repetitive and incorrect log lines on witness verify #317

[ChaosInTheCRD]

What this PR does / why we need it

some cleanup around in-toto/go-witness#317.

• It seemed as though we might be adding the memory source even if we don't specify an attestation-file-path so I have cleaned it up a bit.
• improved the general logging after verify.

testing instructions for @Xopek from in-toto/go-witness#316:

• clone and checkout this branch and the corresponding go-witness branch
• add a replace line to go.mod in this repository to point to your local go-witness repository:

module github.com/in-toto/witness

go 1.22.0

toolchain go1.22.2

++ replace github.com/in-toto/go-witness => <path/to/local/go-witness>

• run go mod tidy
• test with go run . run and go run . verify in the root of this branch.

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[Xopek]

Can confirm, that this fixed issue in-toto/go-witness#316.

However for some reason in order for "verify" command to work, i had to regenerate attestations, without it verify failed with error:

ERROR   Verification failed
ERROR   Evidence:
ERROR   Step: build
ERROR   verification failure: no collections found
ERROR   failed to verify policy: policy verification failed