wrapRustcWith: allow --sysroot to be overridden

It turns out that unlike a normal Unix program, if the --sysroot
option is given more than once, rustc will error rather than using the
last value given.  Therefore, we need to ensure we only add our
default --sysroot argument if one hasn't been given explicitly on the
wrapper's command line.

This fixes cross compilation of rustc.

Closes: https://github.com/NixOS/nixpkgs/issues/271736
Fixes: 8b51cdd3be ("rustc: add a compiler wrapper")

pkgs/build-support/rust/rustc-wrapper/rustc-wrapper.sh

@@ -1,6 +1,19 @@
 #!@shell@
 
-extraBefore=(@sysroot@)
+defaultSysroot=(@sysroot@)
+
+for arg; do
+    case "$arg" in
+        --sysroot)
+            defaultSysroot=()
+            ;;
+        --)
+            break
+            ;;
+    esac
+done
+
+extraBefore=("${defaultSysroot[@]}")
 extraAfter=($NIX_RUSTFLAGS)
 
 # Optionally print debug info.