Use rust-lld on windows rustdoc in config_fast_builds.toml (#13553)

# Objective

- Rustdoc doesn't seem to follow cargo's `linker` setting
- Improves the situation in #12207

## Solution

- Explicitly set the linker in rustdoc flags

## Testing

- I tested this change on Windows and it significantly improves testing
performances (can't give an exact estimate since they got stuck before
this change)

---

Note: I avoided changing the settings on Linux and MacOS because I can't
test on those platforms. It would be nice if someone could test similar
changes there and report so they can be done on all major platforms.

Author: SkiFire13

.cargo/config_fast_builds.toml

@@ -134,6 +134,7 @@ rustflags = [
 # rustup component add llvm-tools
 # ```
 linker = "rust-lld.exe"
+rustdocflags = ["-Clinker=rust-lld.exe"]
 rustflags = [
   # Nightly
   # "-Zshare-generics=n", # This needs to be off if you use dynamic linking on Windows.