envbuilder: address follow-on comments from GetCacheProbe refactor

[johnstcn]

• Extract additional methods from envbuilder Run/RunCacheProbe function extract RunCacheProbe function envbuilder#284 (comment)
• Refactor and extract startStage / endStage to a 'real' function extract RunCacheProbe function envbuilder#284 (comment)
• Use bufio.NewReaderSize when copying output of git clone extract RunCacheProbe function envbuilder#284 (comment)
• Migrate kaniko build logic to use the strategy pattern so it can be replaced in future extract RunCacheProbe function envbuilder#284 (comment)