Document how to set `uv` install directory from `uv-installer.sh`

[SamuelMarks]

Was about to add a feature request then realised it already works:

$ curl --proto '=https' --tlsv1.2 -OLsSf https://github.com/astral-sh/uv/releases/download/0.2.29/uv-installer.sh
$ env -i CARGO_HOME=/tmp/foo/cargo HOME=/tmp/foo/home /usr/bin/bash uv-installer.sh --no-modify-path
downloading uv 0.2.29 x86_64-unknown-linux-gnu
installing to /tmp/foo/cargo/bin
  uv
  uvx
everything's installed!
$ tree -a /tmp/foo
/tmp/foo
├── cargo
│   └── bin
│       ├── uv
│       └── uvx
└── home
    └── .config
        └── uv
            └── uv-receipt.json

6 directories, 3 files

Might be helpful to document this for others.

[zanieb]

Thanks! This would go in https://github.com/astral-sh/uv/blob/main/docs/installation.md#standalone-installer if someone is interested in contributing.