update the WorkDir (-w) flag to default to the Dir (-d) value as documented (#310)
update the WorkDir (-w) flag to default to the Dir (-d) value as documented (#310)

The code settings up the command-line switch was passing `inv.Dir` as the
default value, which at the time of the call is always `"."`. This means
that the later code that sets `inv.WorkDir = inv.Dir` won't run because
`inv.WorkDir` is not the empty string.

The command line -help documentation reads:

      -w <string>
            working directory where magefiles will run (default -d value)

But given the above, this is not the observed behavior.

Now `inv.WorkDir` is defaulted to the empty string, and the code works
as expected.

Co-authored-by: Nate Finch <fb500cd10895b11cf2d94f12ddc0d384aba1275e@gmail.com>
magefile/mage,magefile/mage,magefile/mage