[llvm-lit] Unset command not found in lit internal shell

[Harini0924]

The unset command is currently not found or implemented in the lit internal shell.
Error displayed

# executed command: unset AFL_DRIVER_STDERR_DUPLICATE_FILENAME
# .---command stderr------------
# | 'unset': command not found
# `-----------------------------
# error: command failed with exit status: 127

Files with failures:

• compiler-rt/test/fuzzer/afl-driver-close-fd-mask.test
• compiler-rt/test/fuzzer/afl-driver-stderr.test