cmd/cue/cmd: testscript moved its gopath location

See rogpeppe/go-internal#156;
the leading dot was added to "hide" the directory so that it is not
walked by commands such as `go list ./...`.

This broke our test, as we hard-coded the previous location via
`$WORK/gopath` to get to the default `GOBIN` location.
We can fix the tests and make them future-proof via `$GOPATH` instead.

The trybots didn't catch this on Gerrit as these are long tests.
I had forgotten that the long tests are opt-in,
and so the recent CL that bumped go-internal to v1.9.0 broke master.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I0203595f0352c6205deb22bff3a9a7ca2d06da07
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/542618
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Paul Jolly <paul@myitcv.io>

cmd/cue/cmd/testdata/script/install.txtar

@@ -10,9 +10,9 @@
 env GOPROXY=https://proxy.golang.org
 
 # Sanity check pre install
-! exists $WORK/gopath/bin/cue
+! exists $GOPATH/bin/cue
 
 # Install
 go install cuelang.org/go/cmd/cue@latest
-exec $WORK/gopath/bin/cue help
+exec $GOPATH/bin/cue help
 stdout 'cue evaluates CUE files'

cmd/cue/cmd/testdata/script/install_contributing.txtar

@@ -10,9 +10,9 @@
 env GOPROXY=https://proxy.golang.org
 
 # Sanity check pre install
-! exists $WORK/gopath/bin/git-codereview
+! exists $GOPATH/bin/git-codereview
 
 # Install
 go install golang.org/x/review/git-codereview@latest
-exec $WORK/gopath/bin/git-codereview help
+exec $GOPATH/bin/git-codereview help
 stdout 'Git-codereview is a git helper'