Use more universal windows install instructions (#1811)

Recommend installing uv on windows with

```
powershell -c "irm https://astral.sh/uv/install.ps1 | iex"
```

instead of

```
irm https://astral.sh/uv/install.ps1 | iex
```

to support installing on cmd.exe, the classic non-powershell windows
command prompt.

See axodotdev/cargo-dist#458 for background.
This will also be included in the next cargo-dist release.

I have confirmed this passes on
 * Command Prompt
 * Windows PowerShell
 * PowerShell
 * git bash

Closes #1750

CC @12932 this fixes the uv command prompt installation.

README.md

@@ -37,8 +37,8 @@ Install uv with our standalone installers, or from [PyPI](https://pypi.org/proje
 # On macOS and Linux.
 curl -LsSf https://astral.sh/uv/install.sh | sh
 
-# On Windows (with PowerShell).
-irm https://astral.sh/uv/install.ps1 | iex
+# On Windows.
+powershell -c "irm https://astral.sh/uv/install.ps1 | iex"
 
 # With pip.
 pip install uv