[Buildbot][z/OS] Clear _TAG_REDIR_* vars for llvm-s390x-zos builder (#967)

This PR clears the `_TAG_REDIR_IN`, `_TAG_REDIR_OUT` and `_TAG_REDIR_ERR
`environment variables for the llvm-s390x-zos builder.

This fixes the intermittent `check-clang` failures in the
`clang-format-check-format`. Previously, the worker environment sets
these to txt, which makes z/OS `/bin/sh` tag every pipe and redirect as
text. This interfered with autoconversion (_BPXK_AUTOCVT=ON,
FILETAG(AUTOCVT,AUTOTAG)).

Co-authored-by: Raihaan Sandhu <raihaan.sandhu@ibm.com>