Add version defaults to install scripts

[JakeDawkins]

Fixes #88

Description

This PR changes the two install scripts to behave similarly to each other, each taking the best parts of the other to provide a friendly and intuitive install process for rover :)

Windows changes

Old Behavior
The old installer script only had the ability to download the version it was released with, i.e. the installer released with 0.0.2 could only install rover 0.0.2.

New Behavior
By default, if no VERSION env variable is present, the windows install script will install the version of Rover the corresponding script was released with, similar to before.

The change here is you can now use the $Env:VERSION env variable to override the default version, and download another version of the Rover binary.

Old Behavior
The old installer could only install the rover version defined by the VERSION env variable.

New Behavior
By default, if no VERSION env variable is present, the nix install script will install the version of Rover the corresponding script was released with. Passing the VERSION env variable overrides the default version, and downloads another version of the Rover binary.

No VERSION env variable is needed when executing the script anymore.

---

Fun notes

• I've also tested that appending the corresponding env variable set lines to these files should allow orbiter to set the version at runtime as well :wendyparrot:
• This also fixes the footgun we almost introduced for npm users. If we had made the script check for latest, npm users installing 0.0.3, for example, always would've been installing the latest version (which we definitely would have heard about)

[EverlastingBugstopper]

want to open an issue for updating these hard coded versions as part of our build.rs process?

[lrlna]

nice!

(and thank you for a detailed description, btw!)