[NFC][flang] Fix execute_command_line test for odd environments (#117714)

One of the execute_command_line tests currently runs `cat` on an invalid
file and checks its return value, but since we don't control `cat` or
the user's path, the return value might not be reliably stable on a
per-platform basis. For example, if `git` is installed on Windows in
certain configurations it adds a directory to the path containing a
`cat` with a different set of error codes to the default Windows one.

This patch changes the test to use the `not` binary built by LLVM for
testing purposes, which should always return 1 on any platform
regardless of the user's environment.

0 files changed, 0 insertions, 0 deletions