fix(pkgbuild): honour CARGO_TARGET_DIR in install path

The act_runner exports CARGO_TARGET_DIR=/cache/target for cache
persistence, but the three Rust-app PKGBUILDs hardcoded
target/release/<bin>. Run 87 compiled 8 min then failed at
install stage. Use ${CARGO_TARGET_DIR:-target} so both CI and
local makepkg work.

moongreet-git/PKGBUILD

@@ -40,7 +40,7 @@ build() {
 
 package() {
     cd "$srcdir/greetd-moongreet"
-    install -Dm755 target/release/moongreet "$pkgdir/usr/bin/moongreet"
+    install -Dm755 "${CARGO_TARGET_DIR:-target}/release/moongreet" "$pkgdir/usr/bin/moongreet"
 
     # Greeter config
     install -Dm644 config/moongreet.toml "$pkgdir/etc/moongreet/moongreet.toml"