internal/gomote: update package dependencies

The cpython3 dependency should only be installed on Windows and
Darwin. We can't be sure that the package exists for all other
platforms.

Updates golang/go#63992

Change-Id: I1d1063efc830eb9d347c5e18c0989bc1f5fe727b
Reviewed-on: https://go-review.googlesource.com/c/build/+/542256
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

internal/gomote/swarming.go

diff --git a/internal/gomote/swarming.go b/internal/gomote/swarming.go
index 772308b..11f435f 100644
--- a/internal/gomote/swarming.go
+++ b/internal/gomote/swarming.go
@@ -830,20 +830,21 @@
 	if err != nil {
 		return "", err
 	}
-
 	packages := []*swarmpb.CipdPackage{
 		{Path: "tools/bin", PackageName: "infra/tools/luci-auth/" + cipdPlatform, Version: "latest"},
 		{Path: "tools", PackageName: "golang/bootstrap-go/" + cipdPlatform, Version: "latest"},
-		{Path: "tools", PackageName: "infra/3pp/tools/cpython3/" + cipdPlatform, Version: "latest"},
 	}
 	pythonBin := "python3"
-	if goos == "windows" {
-		pythonBin = `tools\bin\python3.exe`
-	} else if goos == "darwin" {
+	switch goos {
+	case "darwin":
 		pythonBin = `tools/bin/python3`
-		packages = append(packages, &swarmpb.CipdPackage{Path: "tools/bin", PackageName: "infra/tools/mac_toolchain/" + cipdPlatform, Version: "latest"})
+		packages = append(packages,
+			&swarmpb.CipdPackage{Path: "tools/bin", PackageName: "infra/tools/mac_toolchain/" + cipdPlatform, Version: "latest"},
+			&swarmpb.CipdPackage{Path: "tools", PackageName: "infra/3pp/tools/cpython3/" + cipdPlatform, Version: "latest"})
+	case "windows":
+		pythonBin = `tools\bin\python3.exe`
+		packages = append(packages, &swarmpb.CipdPackage{Path: "tools", PackageName: "infra/3pp/tools/cpython3/" + cipdPlatform, Version: "latest"})
 	}
-
 	req := &swarmpb.NewTaskRequest{
 		Name:           name,
 		Priority:       20, // 30 is the priority for builds