Core: Ability to get all environment variables

Describe the enhancement

Because I can not get $HOME from toolkit/exec, but I have many places where I use it. So I want to have a way to get all environment variables in custom actions.

Code Snippet

Add a new function in core.ts.

export function getEnv(name: string): string {
  const val: string = process.env[name.toUpperCase()] || ''
  return val.trim()
}

And I could use it in my custom actions like this.

let HOME_DIR = core.getEnv('HOME')

Start in core.ts, where the issue requests a new environment-variable helper for custom actions. Check how toolkit/exec and existing core utilities expose process values, then verify that the helper returns the requested environment variable in the expected form. Done means custom actions can retrieve values such as HOME through core.