Fix global target dir pollution

This shouldn't be necessary but people override it anyway and it costs
nothing

cargo-cache/PKGBUILD

@@ -23,6 +23,8 @@ prepare() {
 
 build() {
     cd "$pkgname-$pkgver"
+    export RUSTUP_TOOLCHAIN=stable
+    export CARGO_TARGET_DIR=target
     cargo build --release --locked
 }