external commands: Wait for process to finish before exiting (#10100) (#10116)

Issue #10063 points out that cabal exits before the external command
has finished executing. This was a simple oversight to not
waitForProcess on the result of calling createProcess.

This also points out the flaw that there isn't a way for external
commands to signal failure, so we now also propagate the exit code from
the external process.

Fixes #10063

(cherry picked from commit cb57c371)

Co-authored-by: Matthew Pickering <matthewtpickering@gmail.com>