[MINOR] Correct validateAndTransformSchema in GaussianMixture and AFTSurvivalRegression

## What changes were proposed in this pull request?

The line SchemaUtils.appendColumn(schema, $(predictionCol), IntegerType) did not modify the variable schema, hence only the last line had any effect. A temporary variable is used to correctly append the two columns predictionCol and probabilityCol.

## How was this patch tested?

Manually.

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Author: Cédric Pelvet <cedric.pelvet@gmail.com>

Closes #18980 from sharp-pixel/master.