[Github Action] Clean up space before running backwards compatibility test (#5977)

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*Motivation*

Backwards compatibility test always failed with
`Docker environment should have more than 2GB free disk space`.

*Modifications*

Clean up space before running test.
1 file changed
tree: 999965d69bdb24a482053bbc39b7abc16aa674ae
  1. distribution/
  2. docker/
  3. tests/