make sure installer only creates directories in DESTDIR

Fixes rust-lang#80238

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/bootstrap/install.rs

@@ -73,12 +73,7 @@ fn install_sh(
     let docdir_default = datadir_default.join("doc/rust");
     let libdir_default = PathBuf::from("lib");
     let mandir_default = datadir_default.join("man");
-    let prefix = builder.config.prefix.as_ref().map_or(prefix_default, |p| {
-        fs::create_dir_all(p)
-            .unwrap_or_else(|err| panic!("could not create {}: {}", p.display(), err));
-        fs::canonicalize(p)
-            .unwrap_or_else(|err| panic!("could not canonicalize {}: {}", p.display(), err))
-    });
+    let prefix = builder.config.prefix.as_ref().unwrap_or(&prefix_default);
     let sysconfdir = builder.config.sysconfdir.as_ref().unwrap_or(&sysconfdir_default);
     let datadir = builder.config.datadir.as_ref().unwrap_or(&datadir_default);
     let docdir = builder.config.docdir.as_ref().unwrap_or(&docdir_default);
@@ -103,6 +98,13 @@ fn install_sh(
     let libdir = add_destdir(&libdir, &destdir);
     let mandir = add_destdir(&mandir, &destdir);
 
+    let prefix = {
+        fs::create_dir_all(&prefix)
+            .unwrap_or_else(|err| panic!("could not create {}: {}", prefix.display(), err));
+        fs::canonicalize(&prefix)
+            .unwrap_or_else(|err| panic!("could not canonicalize {}: {}", prefix.display(), err))
+    };
+
     let empty_dir = builder.out.join("tmp/empty_dir");
 
     t!(fs::create_dir_all(&empty_dir));