Use getenv instead of gdb_environ on mi-cmd-env.c

Use getenv instead of gdb_environ on mi-cmd-env.c

This is a spinoff of
<https://sourceware.org/ml/gdb-patches/2017-06/msg00437.html>.
mi-cmd-env.c is using the whole gdb_environ machinery in order to
access just one variable, which can be easily replaced by a simple
call to getenv.  This patch does that, and doesn't cause regressions.

gdb/ChangeLog:
2017-06-18  Sergio Durigan Junior  <sergiodj@redhat.com>

	* mi/mi-cm-env.c (_initialize_mi_cmd_env): Use getenv instead of
	gdb_environ to access an environment variable.