fix: skip CLI download in build.rs when DOCS_RS env var is set (#1660)

* Initial plan

* fix: skip CLI download in build.rs when DOCS_RS env var is set

docs.rs builds in a sandboxed environment without network access.
The build.rs script was failing because it tried to download the
Copilot CLI binary. Detect the DOCS_RS environment variable (set
automatically by docs.rs) and skip the download, similar to the
existing COPILOT_SKIP_CLI_DOWNLOAD escape hatch.

Co-authored-by: edburns <75821+edburns@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: edburns <75821+edburns@users.noreply.github.com>
Co-authored-by: Ed Burns <edburns@microsoft.com>

Author: Copilot

rust/build.rs

@@ -5,6 +5,7 @@ use std::time::Duration;
 use sha2::Digest;
 
 fn main() {
+    println!("cargo:rerun-if-env-changed=DOCS_RS");
     println!("cargo:rerun-if-env-changed=COPILOT_SKIP_CLI_DOWNLOAD");
     println!("cargo:rerun-if-env-changed=COPILOT_CLI_EXTRACT_DIR");
     println!("cargo:rerun-if-env-changed=BUNDLED_CLI_CACHE_DIR");
@@ -43,6 +44,13 @@ fn main() {
         return;
     }
 
+    // docs.rs builds in a sandboxed environment without network access.
+    // Skip the CLI download so documentation can be generated successfully.
+    if std::env::var_os("DOCS_RS").is_some() {
+        println!("cargo:warning=DOCS_RS is set — skipping CLI download/bundle/cache");
+        return;
+    }
+
     let Some(platform) = target_platform() else {
         println!("cargo:warning=Unsupported target platform for Copilot CLI bundling — skipping");
         return;