cmd/cue: make long tests pass with Go 1.18

Go 1.18 no longer allows using "go get" to install programs,
as that is now done via "go install" instead.
We already updated the install instructions,
but some of the tests still used "go get" that way.
We require Go 1.16+, so we can assume "go install pkg@version" works.

Also stop setting GO111MODULE=on in a couple of tests,
as that has been the default since Go 1.16.

Finally, since we require Go 1.16 or later, we can simplify the tests to
no longer need to worry about older Go versions.

Updates #1619.

Change-Id: I59b16f3270b20e93efbab4b4044eb33c0ebc67c0
Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/536181
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@gmail.com>

Author: mvdan

cmd/cue/cmd/testdata/script/get_go_bad_embed.txt

@@ -1,7 +1,6 @@
 # Test that we get a sensible error message when there is a type
 # checking error in the package that is being 'cue get go'-ed.
 
-[!go1.16] skip 'Only relevant for Go 1.16 and above'
 [golang.org/issue/44287] skip
 
 cue get go --local

cmd/cue/cmd/testdata/script/get_go_unresolvable_package.txt

@@ -1,12 +1,8 @@
 # Test that we get expected error when we ask cue get go to get
 # a Go package that cannot be resolved.
 
-# Ensure that we don't automatically add the missing dependency pre Go 1.16
-[!go1.16] env GOFLAGS=-mod=readonly
-
 ! cue get go k8s.io/api/apps/v1
-[go1.16] stderr '\Qno required module provides package k8s.io/api/apps/v1'
-[go1.14] [!go1.16] stderr '\Qcannot find module providing package k8s.io/api/apps/v1: import lookup disabled by -mod=readonly'
+stderr '\Qno required module provides package k8s.io/api/apps/v1'
 
 -- go.mod --
 module mod.com/blah

cmd/cue/cmd/testdata/script/install.txt

@@ -1,9 +1,8 @@
-# Test that we can install CUE via the pre Go 1.16 install
-# method (which should also work with Go 1.16) described in
-# the project README
+# Test that we can install CUE via the install method described in the project
+# README, which requires Go 1.16 or later.
 #
 # Keep this test in sync with all places that describe install
-# instructions
+# instructions.
 
 [!long] skip 'Install tests use the network and take some time'
 
@@ -14,7 +13,6 @@ env GOPROXY=https://proxy.golang.org
 ! exists $WORK/gopath/bin/cue
 
 # Install
-env GO111MODULE=on
-go get cuelang.org/go/cmd/cue
+go install cuelang.org/go/cmd/cue@latest
 exec $WORK/gopath/bin/cue help
 stdout 'cue evaluates CUE files'

cmd/cue/cmd/testdata/script/install_contributing.txt

@@ -1,6 +1,5 @@
 # Test that we can install tools required to contribute to the CUE
-# project using the pre Go 1.16 install method (which should also
-# work with Go 1.16)
+# project via the new Go 1.16 method
 #
 # Keep this test in sync with all places that describe install
 # instructions for contributing
@@ -14,7 +13,6 @@ env GOPROXY=https://proxy.golang.org
 ! exists $WORK/gopath/bin/git-codereview
 
 # Install
-env GO111MODULE=on
-go get golang.org/x/review/git-codereview
+go install golang.org/x/review/git-codereview@latest
 exec $WORK/gopath/bin/git-codereview help
 stdout 'Git-codereview is a git helper'