[devbox] Default to the shell already being used (#2478)

## Summary
`devbox shell` now uses the `$SHELL` environment variable to start the same shell that the user is already using. Exported additional environment variables that make it possible to configure a prompt when inside a devbox shell.

## How was it tested?
Compiled and ran on example project.

## Is this change backwards-compatible?
Yes

Author: loreto

boxcli/shell.go

@@ -24,5 +24,6 @@ func runShellCmd(cmd *cobra.Command, args []string) error {
 		return errors.WithStack(err)
 	}
 
+	// TODO: If we're inside a devbox shell already, don't re-run.
 	return box.Shell()
 }

nix/nix.go

@@ -11,6 +11,11 @@ import (
 
 func Shell(path string) error {
 	cmd := exec.Command("nix-shell")
+	// Default to the shell already being used.
+	shell := os.Getenv("SHELL")
+	if shell != "" {
+		cmd.Args = append(cmd.Args, "--command", shell)
+	}
 	cmd.Stdin = os.Stdin
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr

tmpl/shell.nix.tmpl

@@ -5,6 +5,7 @@ let
   }) {};
 in with pkgs;
 mkShell {
+  shellHook = "export name=devbox IN_NIX_SHELL=0; DEVBOX_SHELL_ENABLED=1;";
   packages = [
   {{- range .Packages}}
     {{.}}