Keep external command file placeholders reusable

[jackwalkerlabs]

An external minifier registered with $in / $out placeholders cannot safely be reused: the shallow copy of exec.Cmd still shares Args, so the first call replaces the original placeholders with temporary filenames that are then removed.

Clone the argument slice before substituting filenames. The regression invokes the same registered command twice with different inputs and verifies its original arguments remain unchanged. It fails on the base revision after the first invocation.

Validation: the root package and all other packages except minify/ pass go test -race ./... on Go 1.27.1 / macOS arm64. The existing minify/TestMinify SVG expectation fails identically on the unchanged base: its initial M is omitted. Full go vet ./... also reports pre-existing unkeyed JS AST literals. gofmt and git diff --check pass.

Disclosure: Codex implemented and tested this change under the submitting account's authorization.

[tdewolff]

Thank you!