build: add busybox support to `time` prereq-check

build: add busybox support to `time` prereq-check

Busybox time supports the GNU time '-f' syntax used by the build time
logging implemented in ff6e62b288c, however the prerequisite check added
only works with GNU time installed as `time` or `gtime`.

As busybox is a multicall binary, the name of the symlink setup by
SetupHostCommand also must be changed from `gtime` to `time` to fix the
value of argv[0]. This causes a number of shells (including bash) to use
their builtin impelementation of time, so the sole invocation has been
changed to use `env time` to use the value found on the $PATH.

Signed-off-by: Andy Boyett <agb@agb.io>