Change PreviousFlakePercentage value type from int to float

PreviousFlakePercentage field in SippyTestInfo struct should be
float instead of int given percentage value can be decimal. The data
from Sippy does return previous_flake_percentage as decimal.

Signed-off-by: Vu Dinh <vudinh@outlook.com>

tools/codegen/pkg/sippy/json_types.go

@@ -34,7 +34,7 @@ type SippyTestInfo struct {
 	PreviousFlakes            int         `json:"previous_flakes"`
 	PreviousPassPercentage    float64     `json:"previous_pass_percentage"`
 	PreviousFailurePercentage float64     `json:"previous_failure_percentage"`
-	PreviousFlakePercentage   int         `json:"previous_flake_percentage"`
+	PreviousFlakePercentage   float64     `json:"previous_flake_percentage"`
 	PreviousWorkingPercentage float64     `json:"previous_working_percentage"`
 	PreviousRuns              int         `json:"previous_runs"`
 	NetFailureImprovement     float64     `json:"net_failure_improvement"`