[write-config] Add rust_api_level_cfg_flags.txt

Includes the needed rustflags to fix workflow breakages introduced
by api-level-conditional features (fuchsia_api_level_at_least).

Test: built and ran locally and verified that `fargo check` worked for
netstack3.

Change-Id: Ibf9d0a69bc10a0f5606365c7000e788e4990ba1b
Reviewed-on: https://fuchsia-review.googlesource.com/c/fargo/+/1045952
Reviewed-by: Bruno Dal Bo <brunodalbo@google.com>

src/lib.rs

diff --git a/src/lib.rs b/src/lib.rs
index e4d28f2..b2845c5 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -30,7 +30,7 @@
 };
 use manifest::Manifest;
 
-use anyhow::{anyhow, bail, Error, Context as _};
+use anyhow::{anyhow, bail, Context as _, Error};
 use std::fs;
 use std::fs::File;
 use std::io::Write;
@@ -428,6 +428,10 @@
             get_user_libc_path(target_options)?.to_string_lossy()
         ),
         format!("-Lnative={}", shared_libraries_path(target_options)?.to_string_lossy()),
+        format!(
+            "@{}/rust_api_level_cfg_flags.txt",
+            target_out_dir(target_options.config)?.to_string_lossy()
+        ),
     ];
 
     if get_triple_cpu(target_options) == "aarch64" {