Make --dry-run more useful when download-rustc is enabled

Previously, it would always treat download-rustc as set to false, which made bootstrap issues with
download-rustc hard to debug.

Author: jyn514

src/bootstrap/config.rs

@@ -1813,11 +1813,11 @@ impl Config {
         self.download_rustc_commit().is_some()
     }
 
-    pub(crate) fn download_rustc_commit(&self) -> Option<&'static str> {
+    pub(crate) fn download_rustc_commit(&self) -> Option<&str> {
         static DOWNLOAD_RUSTC: OnceCell<Option<String>> = OnceCell::new();
         if self.dry_run() && DOWNLOAD_RUSTC.get().is_none() {
             // avoid trying to actually download the commit
-            return None;
+            return self.download_rustc_commit.as_deref();
         }
 
         DOWNLOAD_RUSTC

src/bootstrap/download.rs

@@ -402,6 +402,10 @@ impl Config {
 
     fn ci_component_contents(&self, stamp_file: &str) -> Vec<String> {
         assert!(self.download_rustc());
+        if self.dry_run() {
+            return vec![];
+        }
+
         let ci_rustc_dir = self.out.join(&*self.build.triple).join("ci-rustc");
         let stamp_file = ci_rustc_dir.join(stamp_file);
         let contents_file = t!(File::open(&stamp_file), stamp_file.display().to_string());