installation.md: update shell syntax for setting env vars (#7138)

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.

docs/getting-started/installation.md

@@ -60,7 +60,7 @@ Request a specific version by including it in the URL:
  disable this behavior, set `INSTALLER_NO_MODIFY_PATH=1`. For example:
 
  ```
- INSTALLER_NO_MODIFY_PATH=1 bash -c 'curl -LsSf https://astral.sh/uv/install.sh | sh'
+ curl -LsSf https://astral.sh/uv/install.sh | env INSTALLER_NO_MODIFY_PATH=1 sh
  ```
 
 ### PyPI