gotooltest: move default GOPATH out of the main module

Otherwise, commands like `go mod tidy` will try to walk into the default
GOPATH directory. At best, that's a waste of resources, since
directories like GOMODPATH may contain lots of files. At worst, it
results in errors, as directories in GOMODPATH are not valid import paths:

	--- FAIL: TestScripts/list (0.01s)
		testscript.go:413:
			[...]
			> go mod tidy
			[stderr]
			mod/gopath/pkg/mod/fruit.com@v1.1.0/fruit: import path "mod/gopath/pkg/mod/fruit.com@v1.1.0/fruit" should not have @Version

We add the test in goproxytest, as goproxytest already uses gotooltest,
and we need an external module to reproduce the error.

goproxytest/testdata/list.txt

@@ -7,6 +7,13 @@ stdout 'v1.0.0 v1.1.0'
 go get -d fruit.com@v1.0.0
 go get -d fruit.com@v1.1.0
 
+# On Go 1.18 and later, this will error out if GOPATH is ${WORK}/gopath,
+# as that places GOMODCACHE inside the Go module and "mod tidy" walks it.
+# "mod tidy" then complains about invalid import paths such as
+# "mod/gopath/pkg/mod/fruit.com@v1.1.0/fruit".
+# It's for that reason that we moved the default GOPATH to ${WORK}/.gopath.
+go mod tidy
+
 -- go.mod --
 module mod
 

gotooltest/setup.go

@@ -110,7 +110,7 @@ func goEnviron(env0 []string) []string {
 	env := environ(env0)
 	workdir := env.get("WORK")
 	return append(env, []string{
-		"GOPATH=" + filepath.Join(workdir, "gopath"),
+		"GOPATH=" + filepath.Join(workdir, ".gopath"),
 		"CCACHE_DISABLE=1", // ccache breaks with non-existent HOME
 		"GOARCH=" + runtime.GOARCH,
 		"GOOS=" + runtime.GOOS,