env: do not fail on non-existing env.in file

If include/generated/env.in does not exist, which is a typical case for
clean build, quiet_cmd_gen_envp command tries to delete this file
unconditionally.

This produces following warning during the build:
  ENVP    include/generated/env.in
rm: cannot remove 'include/generated/env.in': No such file or directory

Add '-f' option to the `rm` command to not complain if file does not
exist.

Fixes: f432eb6d ("env: Avoid using a leftover text-environment file")
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>