installation.md: update shell syntax for setting env vars

[ilyagr]

I find this more readable; it also no longer requires bash (thought just replacing bash with sh would achieve that much).

Strictly speaking, env is not necessary on most shells, but I find it makes things clearer.

Test Plan

Ran the command locally, did not try compiling the docs.

---

Aside, loosely related (and hopefully helpful) suggestions:

I'm hoping you will also explain in the docs how to install to ~/.local/bin, with the same goal as #6839. Using environment variables for that is fine.

Another minor FR I'd have is to mention these environment variables in the help message of the installer script, especially if you want to encourage people to use them.

Thank you for working on the installation script! It helps me feel more comfortable about eventually asking that people install uv to compile docs on my project, hopefully helping people who don't have a Python environment already installed.

[zanieb]

Thanks this seems reasonable to me! I recently put up #7107 — #6839 which looked a bit too complicated for users and I've been working with the upstream team to make configuration of the installer simpler / clearer.

Another minor FR I'd have is to mention these environment variables in the help message of the installer script, especially if you want to encourage people to use them.

Can you open an issue for this so we can track it?

[zanieb]

One small problem with this approach is now the Windows recommendation is a different style :/