Fix footgun for Windows users in fast_config.toml (#14103)

# Objective

- https://github.com/bevyengine/bevy/pull/13136 changed the default
value of `-Zshare-generics=n` to `-Zshare-generics=y` on Windows
- New users are encouraged to use dynamic builds and LLD when setting up
Bevy
- New users are also encouraged to check out `fast_config.toml`
- https://github.com/bevyengine/bevy/issues/1126 means that running
dynamic builds, using LLD and sharing generics on Windows results in a
cryptic error message

As a result, a new Windows user following all recommendations for better
compiles is actually not able to compile Bevy at all.

## Solution

- Set `-Zshare-generics=n` on Windows with a comment saying this is for
dynamic linking

## Testing

I verified that https://github.com/bevyengine/bevy/issues/1126 is still
in place on the current nightly (1.80)

## Additional Info

Maybe the website should mention this as well? The relevant snippet
there looks like this:
```toml
# /path/to/project/.cargo/config.toml
[target.x86_64-unknown-linux-gnu]
rustflags = [
  # (Nightly) Make the current crate share its generic instantiations
  "-Zshare-generics=y",
]
```
so it kinda implies it's only for Linux? Which is not quite true, this
setting works on macOS as well AFAIK

.cargo/config_fast_builds.toml

@@ -136,7 +136,7 @@ rustflags = [
 linker = "rust-lld.exe"
 rustflags = [
   # Nightly
-  # "-Zshare-generics=y",
+  # "-Zshare-generics=n", # This needs to be off if you use dynamic linking on Windows.
   # "-Zthreads=0",
 ]