feat: allow overriding install path

[mistydemeo]

This provides a way to override any configuration on the installer and install to a location specified by a new environment variable. It provides a more predictable way to insist that it gets installed to a specific place, regardless of how the installer was built.

Possible fix for axodotdev/axoupdater#32, where we'd like to be able to install a new version to exactly the same path as the previous version.

[Gankra]

is there any reason to do this an env-var and not a cli flag..? i suppose for curl-sh it's harder

[mistydemeo]

This also strikes me as a power user/internal feature, which feels better-suited to being an environment variable instead of a CLI flag.

[Gankra]

It is a power-user feature, but it's also an end-power-user feature, right? like they won't even know what "cargo-dist" is?

[mistydemeo]

I don't think end users should use this; this is a feature for the updater which needs fine-grained control over installation location. That other people can use it is kind of a bonus.

[Gankra]

oh I thought this was also motivated by some other astral asks to let the end-user install to a custom dir...

[mistydemeo]

Oh, I think I totally missed that side of the conversation.

[mistydemeo]

One more factor: the shell installer errors on unrecognized flags. An environment variable helps us guard against the possibility of using an installer version that doesn't recognize the new flag, because it'll silently ignore the instruction instead of hard-erroring and not performing the install. eg

$ ./axolotlsay-installer.sh --force-install-prefix /tmp/axolotlsay
ERROR: unknown option --force-install-prefix

[Gankra]

one issue but basic idea seems fine