sql/schemachanger: setColumnType operations in schemachange/random-load will occasionally fail

[ajstorm]

When running the random schema changer workload (schemachange/random-load) the setColumnType operation will occasionally fail complaining that the operation is "not supported inside a transaction". This is strange because the workload specifically checks to see that the schema change operation is non-trivial, and if so, adds that as an acceptable error. It would appear as though some schema change operations are being flagged as non-trivial in the schema change code, but not in the workload. Note that this is possible with schema changes which are provided an expression (due to the following code snippet), but that doesn't seem to be happening in the attached example.

	var kind schemachange.ColumnConversionKind
	if t.Using != nil {
		// If an expression is provided, we always need to try a general conversion.
		// We have to follow the process to create a new column and backfill it
		// using the expression.
		kind = schemachange.ColumnConversionGeneral
	} else {
		kind, err = schemachange.ClassifyConversion(ctx, col.GetType(), typ)
		if err != nil {
			return err
		}
	}

For the time being, I've disabled setColumnType operations in the schemachange/random-load and will use this as a tracking issue.
run_132543.970_n1_workload_run_schemachange.log

Jira issue: CRDB-8168

Epic CRDB-25314