Fix: using putenv() and free()-ing the value is invalid

putenv() does not copy the string passed as the parameter. Hence,
free()-ing the string results in an invalid environment. In the
"good" case, we don't care since we execl().

However, on error, our process now has an invalid environment
which can cause breakage further down the line.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>