Allow customization of install location using standalone installer via an environment variable

[jonathandung]

Currently, the standalone installer script searches through some XDG_* environment variables and falls back to some HOME-related path to install the final executable. Maybe there's another way that I don't know of, but I believe having a dedicated prek-specific environment variable at the highest precedence, something like UV_INSTALL_DIR for uv, would be beneficial.

[j178]

PREK_INSTALL_DIR is available, I just documented it in #2859

[jonathandung]

I think you meant #2859, but thanks!