vpython: make sure that venv falls before system python in path when executing vpython.

Bug: b/491263752
Change-Id: I48995ea1118dd1346bf6d7422fab616a6a6a6964
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/7929928
Commit-Queue: Alex Ovsienko <ovsienko@google.com>
Reviewed-by: Richard Wang <richardwa@google.com>

vpython/application/application.go

diff --git a/vpython/application/application.go b/vpython/application/application.go
index ddb28dd..0c3a8ab 100644
--- a/vpython/application/application.go
+++ b/vpython/application/application.go
@@ -447,6 +447,17 @@
 	env := environ.New(a.Environments)
 	python.IsolateEnvironment(&env)
 
+	// Prepend venv path in the path to make sure system python
+	// will not get executed.
+	if a.PythonExecutable != "" {
+		venvDir := filepath.Dir(a.PythonExecutable)
+		if curPath, ok := env.Lookup("PATH"); ok {
+			env.Set("PATH", strings.Join([]string{venvDir, curPath}, string(os.PathListSeparator)))
+		} else {
+			env.Set("PATH", venvDir)
+		}
+	}
+
 	cl := a.PythonCommandLine.Clone()
 	cl.AddSingleFlag("s")