Run shellcheck on Jenkins scripts (#18874)
Summary:
closes #18873
Doesn't fail the build on warnings yet.
Also fix most severe shellcheck warnings
Limited to `.jenkins/pytorch/` at this time
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18874
Differential Revision: D14936165
Pulled By: kostmo
fbshipit-source-id: 1ee335695e54fe6c387ef0f6606ea7011dad0fd4
