better concurrency support on Linux

[joshdabosh]

What this PR does / why we need it

• When ptracing programs, restrict the product attestor to only attest for files recorded as "opened" by the commandrun attestor. Then, files created by a separate process running concurrently will not be mistakenly included in the attestation products (this happens currently).
• Keep track of newly created files when tracing with ptrace. Previously, they would not be added to the OpenedFiles list because the file would not exist when attempting to calculate its digest.

Which issue(s) this PR fixes (optional)

Acceptance Criteria Met

• Docs changes if needed
• Testing changes if needed
• All workflow checks passing (automatically enforced)
• All review conversations resolved (automatically enforced)
• DCO Sign-off

Special notes for your reviewer:

[mikhailswift]

Hi @joshdabosh, thanks for this contribution! I'll start testing this and give you a review shortly.

[mikhailswift]

Just the one minor nit, otherwise I think this is good to go!