Add nix package manager detector and error if not installed (#2641)

## Summary
Check if nix package manager is installed before calling nix

## How was it tested?
`go build -o devbox cmd/devbox/main.go`
`devbox shell`

## Is this change backwards-compatible?
Yes

nix/nix.go

@@ -6,13 +6,18 @@ 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")