add helpful env vars during shell + run

Author: Lagoja

internal/impl/devbox.go

@@ -764,6 +764,10 @@ func (d *Devbox) computeNixEnv(ctx context.Context, usePrintDevEnvCache bool) (m
 	// Append variables from current env if --pure is not passed
 	currentEnv := os.Environ()
 	env, err := d.parseEnvAndExcludeSpecialCases(currentEnv)
+	env["DEVBOX_PROJECT_ROOT"] = d.projectDir
+	env["DEVBOX_CONFIG_DIR"] = d.projectDir + "/devbox.d"
+	env["DEVBOX_PACKAGES_DIR"] = d.projectDir + "/.devbox/virtenv/.wrappers"
+
 	if err != nil {
 		return nil, err
 	}