devbox: add nix installation check to all commands (#2649)

## Summary
devbox: add nix installation check to all commands

## How was it tested?
go build
devbox shell

## Is this change backwards-compatible?
Yes

Author: LucilleH

boxcli/root.go

@@ -5,7 +5,9 @@ package boxcli
 
 import (
 	"context"
+	"errors"
 	"os"
+	"os/exec"
 
 	"github.com/spf13/cobra"
 )
@@ -17,6 +19,13 @@ func RootCmd() *cobra.Command {
 			// Don't display 'usage' on application errors.
 			cmd.SilenceUsage = true
 		},
+		PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
+			_, err := exec.LookPath("nix-shell")
+			if err != nil {
+				return errors.New("Could not find nix in your PATH\nInstall nix by following the instructions at https://nixos.org/download.html and make sure you've set up your PATH correctly.")
+			}
+			return nil
+		},
 		RunE: func(cmd *cobra.Command, args []string) error {
 			return cmd.Help()
 		},

nix/nix.go

@@ -6,18 +6,13 @@ package nix
 import (
 	"bytes"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"os"
 	"os/exec"
 	"strings"
 )
 
 func Shell(path string) error {
-	_, err := exec.LookPath("nix-shell")
-	if err != nil {
-		return errors.New("Could not find nix in your PATH\nInstall nix by following the instructions at https://nixos.org/download.html and make sure you've set up your PATH correctly.")
-	}
 	cmd := exec.Command("nix-shell", path)
 	// Default to the shell already being used.
 	shell := os.Getenv("SHELL")