do not write sccache stats to json if missing OUR_GITHUB_JOB_ID (#79541)
Allows use of .jenkins/pytorch/build.sh without assuming OUR_GITHUB_JOB_ID is set. This is a regression caused by #79366.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79541
Approved by: https://github.com/suo, https://github.com/seemethere
