Inject UID env variable

This is usually provided by bash, there might be a bunch of those we
want to add in the future (or not).

nosh.go

@@ -3,6 +3,7 @@ package main
 import (
 	"fmt"
 	"os"
+	"os/user"
 
 	noshOs "github.com/Rafflesiaceae/nosh/os"
 
@@ -71,6 +72,10 @@ func run(scriptPath string, src interface{}) {
 		"write":    fs.ModuleWrite,
 	}
 
+	// Injected envs
+	user, _ := user.Current()
+	os.Setenv("UID", user.Uid) // @TODO move to os/user.go
+
 	thread := &starlark.Thread{
 		Name: "nosh",
 		Print: func(thread *starlark.Thread, msg string) {