Allow entrypoint binary to wait for multiple files

[imjasonh]

Changes

Enables the entrypoint binary to be passed multiple comma-separated paths to its -wait_file flag. The binary will wait for all of those paths to exist before proceeding to execute the command.

This is not exposed in the TaskRun API (nor is it expected to be), and the change is fully backward-compatible for existing users of the entrypoint image.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
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• [y] Includes tests (if functionality changed/added)
• [n/a] Includes docs (if user facing)
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Release Notes

entrypoint image allows multiple comma-separated values for the -wait_file flag

[imjasonh]

/assign sbwsg

[ghost]

/lgtm

[ghost]

/test pull-tekton-pipeline-integration-tests

[tekton-robot]

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