Use appropriate default value for sketches-report-path input

[per1234]

In the time since it was first conceived, the use of the report file created by the compile-sketches action has been expanded from only being a way to pass memory usage change data to the arduino/report-size-deltas action to a general purpose report of data generated from the sketch compilations, including:

• Non-deltas size data consumed by arduino/report-size-trends
• Compilation status for each sketch/board combination
• Compiler warning count

It's possible additional information will be added to the report over time.

For this reason, the previous default size-deltas-reports was no longer appropriate and might be the source of confusion.

Unfortunately, this is a breaking change for people who:

• Did not specify a value to the sketches-report-path input and,
• have an additional step in their workflow (e.g., actions/upload-artifact) that relies on the sketches reports being located under the folder resulting from the previous default

[aentinger]

Although a breaking change those are sometimes necessary. As we are currently our own best action customer I agree on doing this breaking change now before the actions get widespread usage. Good work @per1234 🚀