The pnpm home directory should be added to PATH and PNPM_HOME

This change is needed to make the action compatible with pnpm v7.

Related breaking change in pnpm: pnpm/pnpm#4280

src/install-pnpm/run.ts

@@ -1,3 +1,4 @@
+import * as core from '@actions/core'
 import { spawn } from 'child_process'
 import { execPath } from 'process'
 import { join } from 'path'
@@ -22,10 +23,19 @@ export async function runSelfInstaller(inputs: Inputs): Promise<number> {
   const response = await fetch('https://pnpm.js.org/pnpm.js')
   response.body.pipe(cp.stdin)
 
-  return new Promise((resolve, reject) => {
+  const exitCode = await new Promise<number>((resolve, reject) => {
     cp.on('error', reject)
     cp.on('close', resolve)
   })
+  if (exitCode === 0) {
+    const pnpmHome = join(dest, 'node_modules/.bin')
+    core.addPath(pnpmHome)
+    // This environment variable will be available in future steps as PNPM_HOME
+    // It is written as PNPM_HOME_PATH due to a convention in GitHub actions:
+    // > Any new environment variables you set that point to a location on the filesystem should have a _PATH suffix.
+    core.exportVariable('PNPM_HOME_PATH', pnpmHome)
+  }
+  return exitCode
 }
 
 export default runSelfInstaller