(FACT-353) Set LC_ALL environment variable to "C" for child processes.

Adding LC_ALL => C for the with_env call in execute.  This will ensure
that child processes that do not check LANG will output using a C
locale if they respect LC_ALL instead.
Merge pull request #696 from peterhuene/fact/353

(FACT-353) Set LC_ALL environment variable to "C" for child processes.