gotooltest: proxy the new GOMODCACHE env var too

Added in 1.15, it's like GOCACHE but for the module download cache
instead of the build cache.

Also add a minimal test to check that the added env vars look like what
we expect.

A couple of other packages needed to be updated. txtar-addmod did not
support GOMODCACHE yet, and a cmd/testscript test assumed that
GOMODCACHE was always thrown away between test runs.

cmd/testscript/testdata/simple.txt

@@ -1,5 +1,7 @@
 # With .gomodproxy supporting files, any GOPROXY from the
 # environment should be overridden by the test proxy.
+# Note that we want an empty GOMODCACHE, to ensure we have to download modules
+# from our proxy.
 env GOPROXY=0.1.2.3
 
 unquote file.txt
@@ -8,6 +10,7 @@ stdout 'example.com/mod'
 ! stderr .+
 
 -- file.txt --
+>env GOMODCACHE=$WORK
 >go list -m
 >stdout 'example.com/mod'
 >go list fruit.com/...

cmd/txtar-addmod/addmod.go

@@ -20,6 +20,7 @@ package main
 
 import (
 	"bytes"
+	"encoding/json"
 	"flag"
 	"fmt"
 	"io/ioutil"
@@ -100,9 +101,17 @@ func main1() int {
 		return string(out)
 	}
 
-	gopath := strings.TrimSpace(run("go", "env", "GOPATH"))
-	if gopath == "" {
-		fatalf("cannot find GOPATH")
+	var goEnv struct {
+		GOPATH     string
+		GOMODCACHE string
+	}
+	if err := json.Unmarshal([]byte(run("go", "env", "-json", "GOPATH", "GOMODCACHE")), &goEnv); err != nil {
+		fatalf("cannot unmarshal 'go env -json': %v", err)
+	}
+	modCache := goEnv.GOMODCACHE
+	if modCache == "" {
+		// For Go 1.14 and older.
+		modCache = filepath.Join(goEnv.GOPATH, "pkg", "mod")
 	}
 
 	exitCode := 0
@@ -131,13 +140,13 @@ func main1() int {
 		}
 		path = encpath
 
-		mod, err := ioutil.ReadFile(filepath.Join(gopath, "pkg/mod/cache/download", path, "@v", vers+".mod"))
+		mod, err := ioutil.ReadFile(filepath.Join(modCache, "cache", "download", path, "@v", vers+".mod"))
 		if err != nil {
 			log.Printf("%s: %v", arg, err)
 			exitCode = 1
 			continue
 		}
-		info, err := ioutil.ReadFile(filepath.Join(gopath, "pkg/mod/cache/download", path, "@v", vers+".info"))
+		info, err := ioutil.ReadFile(filepath.Join(modCache, "cache", "download", path, "@v", vers+".info"))
 		if err != nil {
 			log.Printf("%s: %v", arg, err)
 			exitCode = 1

gotooltest/setup.go

@@ -27,6 +27,7 @@ var (
 	goEnv struct {
 		GOROOT      string
 		GOCACHE     string
+		GOMODCACHE  string
 		GOPROXY     string
 		goversion   string
 		releaseTags []string
@@ -59,13 +60,14 @@ func initGoEnv() error {
 	eout, stderr, err := run("go", "env", "-json",
 		"GOROOT",
 		"GOCACHE",
+		"GOMODCACHE",
 		"GOPROXY",
 	)
 	if err != nil {
 		return fmt.Errorf("failed to determine environment from go command: %v\n%v", err, stderr)
 	}
 	if err := json.Unmarshal(eout.Bytes(), &goEnv); err != nil {
-		return fmt.Errorf("failed to unmarshal GOROOT and GOCACHE tags from go command out: %v\n%v", err, eout)
+		return fmt.Errorf("failed to unmarshal 'go env -json' output: %v\n%v", err, eout)
 	}
 
 	version := goEnv.releaseTags[len(goEnv.releaseTags)-1]
@@ -138,6 +140,7 @@ func goEnviron(env0 []string) []string {
 		"GOOS=" + runtime.GOOS,
 		"GOROOT=" + goEnv.GOROOT,
 		"GOCACHE=" + goEnv.GOCACHE,
+		"GOMODCACHE=" + goEnv.GOMODCACHE,
 		"GOPROXY=" + goEnv.GOPROXY,
 		"goversion=" + goEnv.goversion,
 	}...)

gotooltest/testdata/env.txt

@@ -0,0 +1,6 @@
+# GOPATH should point inside the temporary work directory, but GOCACHE and
+# GOMODCACHE should not, as they should reuse the host's.
+go env
+stdout GOPATH=.*${WORK@R}
+! stdout GOCACHE=.*${WORK@R}
+! stdout GOMODCACHE=.*${WORK@R}