Use QVM_BIN_DIR fallback for script paths and set in flake

bin/qvm-start

@@ -130,8 +130,8 @@ main() {
         log_info "First run detected - building VM..."
         log_info "This may take several minutes."
         
-        SCRIPT_DIR="$(dirname "$0")"
-        if ! "$SCRIPT_DIR/qvm-rebuild"; then
+        local qvm_rebuild="${QVM_BIN_DIR:-$(dirname "$0")}/qvm-rebuild"
+        if ! "$qvm_rebuild"; then
             die "Failed to build VM. Run 'qvm rebuild' manually to debug."
         fi
     fi