Add addPath to core (#13)

* Add add-path

* Format

* lint

packages/core/__tests__/lib.test.ts

@@ -1,5 +1,6 @@
 import {ExitCode} from '@actions/exit'
 import * as os from 'os'
+import * as path from 'path'
 import * as core from '../src/core'
 
 /* eslint-disable @typescript-eslint/unbound-method */
@@ -11,6 +12,7 @@ const testEnvVars = {
   'my secret': '',
   'special char secret \r\n];': '',
   'my secret2': '',
+  PATH: `path1${path.delimiter}path2`,
 
   // Set inputs
   INPUT_MY_INPUT: 'val',
@@ -78,6 +80,14 @@ describe('@actions/core', () => {
     ])
   })
 
+  it('prependPath produces the correct commands and sets the env', () => {
+    core.addPath('myPath')
+    expect(process.env['PATH']).toBe(
+      `myPath${path.delimiter}path1${path.delimiter}path2`
+    )
+    assertWriteCalls([`##[add-path]myPath${os.EOL}`])
+  })
+
   it('getInput gets non-required input', () => {
     expect(core.getInput('my input')).toBe('val')
   })