Conda installation leaves tmp/installer.sh behind

[oschulz]

Due to

if Sys.isunix()
            installer = joinpath(INSTALLER_DIR, "installer.sh")

we currently leave a file "tmp/installer.sh". This causes trouble on multi-user systems (compute servers, etc.) - one user installs Conda.jl and creates and owns "tmp/installer.sh", then another user tries to install Conda.jl and get's the error that "installer.sh" can't be unlinked (because it's owned by the other user).

We should use a more random filename for the installer script. :-)

[stevengj]

I agree, we should use tempname() here and delete it when we are done.