Make nix-build args copy-pastable via set -x

A reproduce script includes a logline that may resemble:

> using these flags: --arg nixpkgs { outPath = /tmp/build-137689173/nixpkgs/source; rev = "fdc872fa200a32456f12cc849d33b1fdbd6a933c"; shortRev = "fdc872f"; revCount = 273100; } -I nixpkgs=/tmp/build-137689173/nixpkgs/source --arg officialRelease false --option extra-binary-caches https://hydra.nixos.org/ --option system x86_64-linux /tmp/build-137689173/nixpkgs/source/pkgs/top-level/release.nix -A 

These are passed along to nix-build and that's fine and dandy, but you can't just copy-paste this as is, as the `{}` introduces a syntax error and the value accompanying `-A` is `''`.

A very naive approach is to just `printf "%q"` the individual args, which makes them safe to copy-paste. Unfortunately, this looks awful due to the liberal usage of slashes:

```
$ printf "%q" '{ outPath = /tmp/build-137689173/nixpkgs/source; rev = "fdc872fa200a32456f12cc849d33b1fdbd6a933c"; shortRev = "fdc872f"; revCount = 273100; }'
\{\ outPath\ =\ /tmp/build-137689173/nixpkgs/source\;\ rev\ =\ \"fdc872fa200a32456f12cc849d33b1fdbd6a933c\"\;\ shortRev\ =\ \"fdc872f\"\;\ revCount\ =\ 273100\;\ \}
```

Alternatively, if we just use `set -x` before we execute nix-build, we'll get the whole invocation in a friendly, copy-pastable format that nicely displays `{}`-enclosed content and preserves the empty arg following `-A`:

```
running nix-build...
using this invocation: 
+ nix-build --arg nixpkgs '{ outPath = /tmp/build-138165173/nixpkgs/source; rev = "e0e4484f2c028d2269f5ebad0660a51bbe46caa4"; shortRev = "e0e4484"; revCount = 274008; }' -I nixpkgs=/tmp/build-138165173/nixpkgs/source --arg officialRelease false --option extra-binary-caches https://hydra.nixos.org/ --option system x86_64-linux /tmp/build-138165173/nixpkgs/source/pkgs/top-level/release.nix -A ''
```

src/root/reproduce.tt

@@ -210,10 +210,9 @@ if [ -n "$printFlags" ]; then
 fi
 
 info "running nix-build..."
-echo "using these flags: ${args[@]}" >&2
-
-exec nix-build "${args[@]}" "${extraArgs[@]}"
-
+echo "using the following invocation:" >&2
+set -x
+nix-build "${args[@]}" "${extraArgs[@]}"
 }
 
 main "$@"