Avoid attempting rename in copy fallback path (#1546)

## Summary

This _could_ fix #1454, but I'm
not sure. I was able to replicate by forcing a bunch of error states.
But, in short, if we fail to hardlink on the initial copy due to a file
existing, and then fail _again_, we fallback to copying. But if we copy,
then the tempfile doesn't exist, and so the `fs_err::rename(&tempfile,
&out_path)?;` will fail with "File not found".

This PR just ensures that the cases are explicitly mutually exclusive:
we only attempt to rename if the hardlink succeeded.

crates/install-wheel-rs/src/linker.rs

@@ -416,11 +416,12 @@ fn hardlink_wheel_files(
                         // Removing and recreating would lead to race conditions.
                         let tempdir = tempdir_in(&site_packages)?;
                         let tempfile = tempdir.path().join(entry.file_name());
-                        if fs::hard_link(path, &tempfile).is_err() {
+                        if fs::hard_link(path, &tempfile).is_ok() {
+                            fs_err::rename(&tempfile, &out_path)?;
+                        } else {
                             fs::copy(path, &out_path)?;
                             attempt = Attempt::UseCopyFallback;
                         }
-                        fs_err::rename(&tempfile, &out_path)?;
                     } else {
                         fs::copy(path, &out_path)?;
                         attempt = Attempt::UseCopyFallback;