handle last-commit in Config::download_ci_rustc_commit for CI

Signed-off-by: onur-ozkan <work@onurozkan.dev>

Author: onur-ozkan

src/bootstrap/src/core/config/config.rs

@@ -2418,13 +2418,16 @@ impl Config {
         let compiler = format!("{top_level}/compiler/");
         let library = format!("{top_level}/library/");
 
+        // If we are running in CI, the current commit will never have artifacts already built.
+        let tip_commit = if CiEnv::is_ci() { "HEAD^" } else { "HEAD" };
+
         // Look for a version to compare to based on the current commit.
         // Only commits merged by bors will have CI artifacts.
         let merge_base = output(
             self.git()
                 .arg("rev-list")
                 .arg(format!("--author={}", self.stage0_metadata.config.git_merge_commit_email))
-                .args(["-n1", "--first-parent", "HEAD"]),
+                .args(["-n1", "--first-parent", tip_commit]),
         );
         let commit = merge_base.trim_end();
         if commit.is_empty() {