emsdk

Remove extraneous arguments to construct_env (#328)

The code for construct_env expects the output file to be at `argv[2]`,
but it is actually invoked in emsdk_env.{sh,fish} with $@ there
instead. Usually this is not a problem because the emdsk_env.{sh,fish}
is `sourced` directly from the user's shell and $@ is empty, but this
can break in scripted environments.

The $@ was added two years ago in 3b6c6b86 for no discernible reason,
but it was entirely ignored in the code until last month when #307
added a meaningful argument to construct_env but incorrectly kept the
$@.