feat: add output-file option, default to random directory output in temp

[kzantow]

This PR makes a couple changes to the scan-action, one of which may be breaking for users:

• modifies the grype invocation to output to a random file in $TEMP, instead of outputting directly in the workspace
• adds an output-file parameter to allow users to specify an output file explicitly

Fixes #216 Fixes #238

[willmurphyscode]

discussed with @kzantow offline - there are still a few things that might be changed before the merge relating to multiple outputs. I'm moving this back to "in progress" and will re-review later.