cmd/gomobile: make gomobile-init support Go modules

Before this change, all the gomobile commands forced Go modules
to be off internally, regardless of the current Go modules state.

After this change, gomobile-init command follows the current Go
modules state. The other gomobile commands are not changed.

This is also a preparation to support Go modules in gomobile-bind
and gomobile-build.

Updates golang/go#27234
Cherry picked from github.com/golang/mobile

cmd/fyne/internal/mobile/build.go

@@ -127,6 +127,8 @@ func runBuildImpl(cmd *command) (*packages.Package, error) {	cleanup, err := bui
 		if pkg.Name != "main" {
 			for _, arch := range targetArchs {
 				env := androidEnv[arch]
+				// gomobile-build does not support Go modules yet.
+				env = append(env, "GO111MODULE=off")
 				if err := goBuild(pkg.PkgPath, env); err != nil {
 					return nil, err
 				}
@@ -148,6 +150,8 @@ func runBuildImpl(cmd *command) (*packages.Package, error) {	cleanup, err := bui
 		if pkg.Name != "main" {
 			for _, arch := range targetArchs {
 				env := darwinEnv[arch]
+				// gomobile-build does not support Go modules yet.
+				env = append(env, "GO111MODULE=off")
 				if err := goBuild(pkg.PkgPath, env); err != nil {
 					return nil, err
 				}
@@ -351,8 +355,6 @@ func goCmdAt(at string, subcmd string, srcs []string, env []string, args ...stri
 	cmd.Args = append(cmd.Args, args...)
 	cmd.Args = append(cmd.Args, srcs...)
 	cmd.Env = append([]string{}, env...)
-	// gomobile does not support modules yet.
-	cmd.Env = append(cmd.Env, "GO111MODULE=off")
 	cmd.Dir = at
 	return runCmd(cmd)
 }

cmd/fyne/internal/mobile/build_androidapp.go

@@ -68,6 +68,8 @@ func goAndroidBuild(pkg *packages.Package, bundleID string, androidArchs []strin
 
 	for _, arch := range androidArchs {
 		env := androidEnv[arch]
+		// gomobile-build does not support Go modules yet.
+		env = append(env, "GO111MODULE=off")
 		toolchain := ndk.Toolchain(arch)
 		libPath := "lib/" + toolchain.abi + "/lib" + libName + ".so"
 		libAbsPath := filepath.Join(tmpdir, libPath)

cmd/fyne/internal/mobile/build_iosapp.go

@@ -65,7 +65,10 @@ func goIOSBuild(pkg *packages.Package, bundleID string, archs []string, appName
 	for _, arch := range archs {
 		path := filepath.Join(tmpdir, arch)
 		// Disable DWARF; see golang.org/issues/25148.
-		if err := goBuild(src, darwinEnv[arch], "-ldflags=-w", "-o="+path); err != nil {
+		env := darwinEnv[arch]
+		// gomobile-build does not support Go modules yet.
+		env = append(env, "GO111MODULE=off")
+		if err := goBuild(src, env, "-ldflags=-w", "-o="+path); err != nil {
 			return nil, err
 		}
 		if nmpkgs == nil {