exec: replace global state with interface

This is cleaner, and finally enables writing tests for the nix invoking functions.

cmd/nixbuild/main.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"log"
 	"os"
 	"os/signal"
@@ -20,7 +21,9 @@ type commandHandlerError string
 func (c commandHandlerError) Error() string { return string(c) }
 
 func main() {
-	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
+	var ctx nixbuild.Context
+
+	nixCtx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
 	defer stop()
 
 	var (
@@ -31,10 +34,13 @@ func main() {
 	c := command.New(os.Stderr, log.Printf, "nixbuild", func(args []string) error {
 		log.SetFlags(0)
 		log.SetPrefix("nixbuild: ")
-		nixbuild.Stdout = os.Stdout
+
+		var stderr io.Writer
 		if flagVerbose {
-			nixbuild.Stderr = os.Stderr
+			stderr = os.Stderr
 		}
+		ctx = nixbuild.New(nixCtx, nil, os.Stdout, stderr)
+
 		return nil
 	}).
 		Flag(&flagNixOS, "nixos", command.BoolFlag(false), "Interpret input as NixOS flake installable").