install-node-and-pnpm does not detect previously installed pnpm

[nils-christian]

Do you want to request a feature or report a bug?

This is a bug similar to #519.

What is the current behavior?

When running install-node-and-pnpm multiple times, it does not detect that pnpm was installed when run previously and instead installs pnpm from scratch each time. See log messages:

[INFO] --- frontend-maven-plugin:1.12.0:install-node-and-pnpm (default-cli) @ test ---
[INFO] Node v14.15.5 is already installed.
[INFO] PNPM 5.18.10 was installed, but we need version v5.18.10
[INFO] Installing pnpm version v5.18.10

What is the expected behavior?

pnpm should only be installed when it is either not installed or the version is actually different.

Please mention your frontend-maven-plugin and operating system version.

The frontend-maven-plugin-Version is 1.12.1, the OS is a windows based system.

[nils-christian]

I created a PR to solve this issue: #1040