[nix] Don't add bad package to devbox.json (#525)

## Summary

If we get any error while trying to add packages we revert devbox.json
and show error.

## How was it tested?

Tried to add a broken package and made sure it did not get added to
devbox.json

```bash
devbox add php81Extensions.blackfire
Installing nix packages.
	error: attribute 'extensionName' missing
	       at /nix/store/j5a7i3dvxslg2ychfy53wdhg1m3xfrwm-source/pkgs/development/interpreters/php/generic.nix:94:29:
	           93|
	           94|           getExtName = ext: ext.extensionName;
	             |                             ^
	           95|
	(use '--show-trace' to show detailed location information)

There was an error installing nix packages: php81Extensions.blackfire. Packages were not added to devbox.json
Error: apply Nix derivation: running command /nix/var/nix/profiles/default/bin/nix-env --profile /Users/mike/dev/scratch/php/.devbox/nix/profile/default --install -f /Users/mike/dev/scratch/php/.devbox/gen/development.nix: exit status 1 with command stderr:
```

Author: mikeland73

internal/impl/devbox.go

@@ -111,6 +111,7 @@ func (d *Devbox) Config() *Config {
 }
 
 func (d *Devbox) Add(pkgs ...string) error {
+	original := d.cfg.Packages
 	// Check packages are valid before adding.
 	for _, pkg := range pkgs {
 		ok := nix.PkgExists(d.cfg.Nixpkgs.Commit, pkg)
@@ -132,6 +133,18 @@ func (d *Devbox) Add(pkgs ...string) error {
 
 	d.pluginManager.ApplyOptions(plugin.WithAddMode())
 	if err := d.ensurePackagesAreInstalled(install); err != nil {
+		// if error installing, revert devbox.json
+		// This is not perfect because there may be more than 1 package being
+		// installed and we don't know which one failed. But it's better than
+		// blindly add all packages.
+		color.New(color.FgRed).Fprintf(
+			d.writer,
+			"There was an error installing nix packages: %v. "+
+				"Packages were not added to devbox.json\n",
+			strings.Join(pkgs, ", "),
+		)
+		d.cfg.Packages = original
+		_ = d.saveCfg() // ignore error to ensure we return the original error
 		return err
 	}