refactor(build-std): skip consulting std_crate

The only functionality of `standard_lib::std_crates` is adding `test` to
the requested crate list. `resolve_std` already unconditionally adds
`sysroot` to the list, which always includes `test` in dependency graph.
Therefore calling `std_crates` is redundant.

src/cargo/ops/cargo_fetch.rs

@@ -64,8 +64,7 @@ pub fn fetch<'a>(
     }
 
     // If -Zbuild-std was passed, download dependencies for the standard library.
-    // We don't know ahead of time what jobs we'll be running, so tell `std_crates` that.
-    if let Some(crates) = standard_lib::std_crates(gctx, None) {
+    if let Some(crates) = gctx.cli_unstable().build_std.as_ref() {
         let (std_package_set, _, _) =
             standard_lib::resolve_std(ws, &mut data, &build_config, &crates)?;
         packages.add_set(std_package_set);