env/linux-x86-alpine: update version of alpine to 3.18

Updates golang/go#39857

Change-Id: Ibb2f27947d9f5f5856862dc78465215964850e95
Reviewed-on: https://go-review.googlesource.com/c/build/+/521975
Auto-Submit: Carlos Amedee <carlos@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

env/linux-x86-alpine/Dockerfile

@@ -8,7 +8,7 @@
 
 FROM golang/buildlet-stage0 AS stage0
 
-FROM alpine:3.16
+FROM alpine:3.18
 MAINTAINER golang-dev <golang-dev@googlegroups.com>
 
 RUN apk add --no-cache \