gopls/integration/govim: build gopls using go1.18rc1

Govim tests expect error messages to use 'any', so build gopls using the
RC.

For golang/go#51056

Change-Id: I0f8cab5d7eea3efbbd5b3f3dd9918e85831c2d50
Reviewed-on: https://go-review.googlesource.com/c/tools/+/388436
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>

gopls/integration/govim/run_local.sh

@@ -13,7 +13,7 @@ Usage: $0 [--sudo] [--short] [--version (semver|latest)]
 Args:
   --sudo     run docker with sudo
   --short    run `go test` with `-short`
-  --version  run on the specific tagged Go version (or latest) rather
+  --version  run on the specific tagged govim version (or latest) rather
              than the default branch
 
 Run govim tests against HEAD using local docker.
@@ -71,7 +71,7 @@ trap "rm -f \"${temp_gopls}\"" EXIT
 ${SUDO_IF_NEEDED}docker run --rm -t \
   -v "${tools_dir}:/src/tools" \
   -w "/src/tools/gopls" \
-  golang:latest \
+  golang:rc \
   go build -o $(basename ${temp_gopls})
 
 # Build the test harness. Here we are careful to pass in a very limited build