CI: fix rustfmt not reporting error on diff
CI: fix rustfmt not reporting error on diff

Due to a change in the rustfmt codebase,
the '--write-mode-diff' no longer reports
error code if code does not match. As a
temporary workaround, we are checking against
the actual reported output to be empty.

Signed-off-by: Diana Popa <4240422768c1b315e51818a074064501037757cd@amazon.com>

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