cmd: ensure that GOPATH is always valid in subprocesses that execute 'go build'

GOPATH/pkg contains, among other things, the module cache (and associated lockfiles).

Fixes #30776

Change-Id: I305cb3c0daab8cedd2e6ad235d4733f66af18723
Reviewed-on: https://go-review.googlesource.com/c/go/+/167082Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/cover/cover_test.go

@@ -82,6 +82,7 @@ func TestMain(m *testing.M) {
 		fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)
 	}
+	os.Setenv("GOPATH", filepath.Join(dir, "_gopath"))
 
 	testTempDir = dir
 

src/cmd/internal/obj/x86/issue19518_test.go

@@ -64,7 +64,8 @@ func objdumpOutput(t *testing.T) []byte {
 		testenv.GoToolPath(t), "build", "-o",
 		filepath.Join(tmpdir, "output"))
 
-	cmd.Env = append(os.Environ(), "GOARCH=amd64", "GOOS=linux")
+	cmd.Env = append(os.Environ(),
+		"GOARCH=amd64", "GOOS=linux", "GOPATH="+filepath.Join(tmpdir, "_gopath"))
 	cmd.Dir = tmpdir
 
 	out, err := cmd.CombinedOutput()

src/cmd/internal/obj/x86/obj6_test.go

@@ -99,7 +99,8 @@ func asmOutput(t *testing.T, s string) []byte {
 		testenv.GoToolPath(t), "tool", "asm", "-S", "-dynlink",
 		"-o", filepath.Join(tmpdir, "output.6"), tmpfile.Name())
 
-	cmd.Env = append(os.Environ(), "GOARCH=amd64", "GOOS=linux")
+	cmd.Env = append(os.Environ(),
+		"GOARCH=amd64", "GOOS=linux", "GOPATH="+filepath.Join(tmpdir, "_gopath"))
 	asmout, err := cmd.CombinedOutput()
 	if err != nil {
 		t.Fatalf("error %s output %s", err, asmout)

src/cmd/link/link_test.go

@@ -161,7 +161,8 @@ TEXT ·x(SB),0,$0
 `)
 	cmd := exec.Command(testenv.GoToolPath(t), "build")
 	cmd.Dir = tmpdir
-	cmd.Env = append(os.Environ(), []string{"GOARCH=amd64", "GOOS=linux"}...)
+	cmd.Env = append(os.Environ(),
+		"GOARCH=amd64", "GOOS=linux", "GOPATH="+filepath.Join(tmpdir, "_gopath"))
 	out, err := cmd.CombinedOutput()
 	if err == nil {
 		t.Fatalf("expected build to fail, but it succeeded")