I faced the same issue mentioned in #1299, this PR fixes the issue by
taking in account specific `set` and `unset` syntax for Fish shell.
The post install instruction (i.e : `./emsdk activate latest`) provides
wrong instructions in Fish shell and Cshell cases.
This should be fixed by this PR too, although the code is a bit
redundant and it could be more cleaner.
Remove the optional argument to contruct_env. It simple if we
always construct the env in the same place. Avoid using temp files,
and avoid changing directory.
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
$@.
