Parse env file discard utf8 bom (#38)

* powershell 5 may add the BOM even when explicitly using utf8

* add test + apply to GITHUB_PATH as well

pkg/runner/testdata/windows-add-env-powershell-5/action/action.yml

@@ -0,0 +1,7 @@
+runs:
+  using: composite
+  steps:
+  - run: |
+      echo $env:GITHUB_ENV
+      echo "kEy=n/a" | Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append
+    shell: powershell