Windows, Bash launcher: Make sure bash bin tool directory is in PATH

If bash_bin_path exists, we add it's directory to PATH to make bash bin tools available.
If bash_bin_path doesn't exist, there are two cases:
   1. bash.exe is in PATH, and that means bash bin tools should also be in PATH.
   2. bash.exe isn't in PATH, the launcher will fail with "The system cannot find the file
      specified." error.

RELNOTES: None
PiperOrigin-RevId: 208182717

src/tools/launcher/bash_launcher.cc

@@ -30,9 +30,15 @@ static constexpr const char* BASH_BIN_PATH = "bash_bin_path";
 
 ExitCode BashBinaryLauncher::Launch() {
   wstring bash_binary = this->GetLaunchInfoByKey(BASH_BIN_PATH);
-  // If specified bash binary path doesn't exist, then fall back to
-  // bash.exe and hope it's in PATH.
-  if (!DoesFilePathExist(bash_binary.c_str())) {
+  if (DoesFilePathExist(bash_binary.c_str())) {
+    wstring bash_bin_dir = GetParentDirFromPath(bash_binary);
+    wstring path_env;
+    GetEnv(L"PATH", &path_env);
+    path_env = bash_bin_dir + L";" + path_env;
+    SetEnv(L"PATH", path_env);
+  } else {
+    // If specified bash binary path doesn't exist, then fall back to
+    // bash.exe and hope it's in PATH.
     bash_binary = L"bash.exe";
   }