[flang] Disable pipefail for tests for stdin input

We have seen the llvm test fail every so often on our bots:
https://lab.llvm.org/buildbot/#/builders/173/builds/6711

This happens because `flang-new -fc1 -S` does not wait for the
pipe to finish being written to/does not read out all the content.
You can see this in the output, cat comes after flang:
+ /home/tcwg-buildbot/worker/flang-aarch64-release/build/bin/not /home/tcwg-buildbot/worker/flang-aarch64-release/build/bin/flang-new -fc1 -S - -o -
+ cat /home/tcwg-buildbot/worker/flang-aarch64-release/llvm-project/flang/test/Driver/input-from-stdin-llvm.ll
error: Invalid input type - expecting a Fortran file

This means that cat gets SIGPIPE which causes it to exit with
code 141 and that's the final result due to pipefail.

flang isn't wrong to exit early (I think some modes of grep also do this)
and we only care about flang's exit code. So disable pipefail for
the stdin testing.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D130619