unix/linux: update to Go 1.18

Change-Id: I663b4e6346cec4a51f4423a38cc1315c5b51956d
Reviewed-on: https://go-review.googlesource.com/c/sys/+/396114
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
diff --git a/unix/linux/Dockerfile b/unix/linux/Dockerfile
index e36d00d..03211b6 100644
--- a/unix/linux/Dockerfile
+++ b/unix/linux/Dockerfile
@@ -21,9 +21,9 @@
 RUN git clone --branch release/2.35/master --depth 1 https://sourceware.org/git/glibc.git
 
 # Get Go
-ENV GOLANG_VERSION 1.18rc1
+ENV GOLANG_VERSION 1.18
 ENV GOLANG_DOWNLOAD_URL https://golang.org/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
-ENV GOLANG_DOWNLOAD_SHA256 9ea4e6adee711e06fa95546e1a9629b63de3aaae85fac9dc752fb533f3e5be23
+ENV GOLANG_DOWNLOAD_SHA256 e85278e98f57cdb150fe8409e6e5df5343ecb13cebf03a5d5ff12bd55a80264f
 
 RUN curl -fsSL "$GOLANG_DOWNLOAD_URL" -o golang.tar.gz \
     && echo "$GOLANG_DOWNLOAD_SHA256  golang.tar.gz" | sha256sum -c - \