Add julia to path immediately

The extra step of reloading the shell or re-executing a startup file seems unnecessary to me.

This is especially important for scripts using juliaup via expect. It would be nice to not have to do shell detection and pick which startup file to execute.

Additionally, some folks' startup files may not behave well when run repeatedly as julaup currently recommends.
```
x@X ~ % curl -fsSL https://install.julialang.org | sh
...
Julia was successfully installed on your system.

Depending on which shell you are using, run one of the following
commands to reload the the PATH environment variable:

. /Users/x/.profile
. /Users/x/.bash_profile
. /Users/x/.zshrc

x@X ~ % julia
zsh: command not found: julia
```

Start by tracing the installer invoked through install.julialang.org and how it updates PATH, with attention to juliaup via expect and the listed shell startup files: .profile, .bash_profile, and .zshrc. Done means Julia is available immediately after installation without shell detection, startup-file reloading, or repeated execution of those files.

Written by the indexing model from the issue text.