Add `$PONYUP_PREFIX` as another option to point ponyup's prefix

[viscropst]

Title.For easier to deploy the development environment and runtime enviroment, user can set this environment variable before running the ponyup or shell and use the ponyup with prefix as $PONYUP_PREFIX until the variable was unset, if user wants to use other than $PONYUP_PREFIX for once, they can use --prefix to change the prefix.

[jemc]

We briefly mentioned this in today's sync call and this makes sense to me. 👍

[SeanTAllen]

What is the specific issue that you ran into that this solves?

[viscropst]

For me, I'm creating and using a tool that setting the every environment I want and run my editor for portable use(eg. setting up a pony environment before running the vscode whatever OS re-installed or change the OS) without virtualization tech.
For persent ponyup I can't using a more better option than modifing the batch file generated by ponyup or wrapping the ponyup batch with --prefix arg when I placed the ponyup prefix out of default place and using this place for daily ponyup use.

[SeanTAllen]

More questions:

• Would you want to have a single ponyup binary that is installed in a single location and then use that with different installation locations for binaries it installed?

OR

• Would you be wanting to install different ponyup binaries in different locations and each of those has its own install location?

[viscropst]

Sorry for late answering.If ponyup acts like rustup's role in ponylang,It's yes to installing the ponyup binary to a single location then use different location for toolchain binaries.

[SeanTAllen]

Would you be installing multiple versions of pony tools in each location or a single version?

[viscropst]

For my preference I would like to install multiple versions of pony tools in each location.

[SeanTAllen]

Rather than overriding prefix, this should be an option to override the default location to look look for/install the ponyp directory. So that the default_root can be overridden and then there's no oddness with interacting with the prefix option.

https://github.com/ponylang/ponyup/blob/main/cmd/main.pony#L17

Note to anyone who works on this, there should be some level of testing around this working so it doesn't get broken in the future.