rust: Fix compile error if build dir and DL_DIR on separate filesystems

The rust bootstrap downloads files into a "tmp" directory then moves the
files into the "cache" directory using std::fs::rename. There are no
issues in the original/unpatched case as "tmp" and "cache" are
subdirectories in the build directory ($(HOST_BUILD_DIR)/build) and so
are nearly guaranteed to be on the same filesystem.

35768bf changed where files are
saved/cached (in $(DL_DIR)/rustc). If HOST_BUILD_DIR and DL_DIR are on
separate filesystems, then using std::fs::rename to move the files will
fail.[1]

This updates 0002-rustc-bootstrap-cache.patch to account for this case,
i.e. if std::fs::rename fails, fall back to copying the file then
removing the original.

[1]: #22457

Fixes: 35768bf ("rust: Cache bootstrap downloads to $(DL_DIR)/rustc")

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

lang/rust/patches/0002-rustc-bootstrap-cache.patch

@@ -11,7 +11,22 @@
                  os.makedirs(rustc_cache)
 --- a/src/bootstrap/download.rs
 +++ b/src/bootstrap/download.rs
-@@ -520,7 +520,10 @@ impl Config {
+@@ -202,7 +202,13 @@ impl Config {
+             Some(other) => panic!("unsupported protocol {other} in {url}"),
+             None => panic!("no protocol in {url}"),
+         }
+-        t!(std::fs::rename(&tempfile, dest_path));
++        match std::fs::rename(&tempfile, dest_path) {
++            Ok(v) => v,
++            Err(_) => {
++                t!(std::fs::copy(&tempfile, dest_path));
++                t!(std::fs::remove_file(&tempfile));
++            }
++        }
+     }
+
+     fn download_http_with_retries(&self, tempfile: &Path, url: &str, help_on_error: &str) {
+@@ -520,7 +526,10 @@ impl Config {
          key: &str,
          destination: &str,
      ) {
@@ -23,7 +38,7 @@
          let cache_dir = cache_dst.join(key);
          if !cache_dir.exists() {
              t!(fs::create_dir_all(&cache_dir));
-@@ -647,7 +650,10 @@ download-rustc = false
+@@ -647,7 +656,10 @@ download-rustc = false
          let llvm_assertions = self.llvm_assertions;
 
          let cache_prefix = format!("llvm-{llvm_sha}-{llvm_assertions}");