R1:241cfc2e8312

Don't leave temporary files around when trying to use credentials with destroyed secrets

Summary: Ref T4284. This fixes at least one problem which can cause the observed behavior.

Test Plan:
- Before applying patch, used `PHABRICATOR_CREDENTIAL=PHID-CDTL-... bin/ssh-connect` + debugging prints to verify the keyfile was written and cleaned up normally.
- Destroyed the credental, verified the temporary file was not cleand up correctly.
- Applied patch, verified temporary file was not written and…