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/push.yml

@@ -6,7 +6,7 @@ jobs:
     steps:
     - run: |
         echo $env:GITHUB_ENV
-        echo "key=val" > $env:GITHUB_ENV
+        echo "key=val" >> $env:GITHUB_ENV
         echo "key2<<EOF" >> $env:GITHUB_ENV
         echo "line1" >> $env:GITHUB_ENV
         echo "line2" >> $env:GITHUB_ENV
@@ -24,7 +24,9 @@ jobs:
         }
     - run: |
         echo $env:GITHUB_ENV
+        # Defect missing append
         echo "KEY=test" > $env:GITHUB_ENV
+        # Defect missing append, test is broken!!!
         echo "Key=expected" > $env:GITHUB_ENV
     - name: Assert GITHUB_ENV is merged case insensitive
       run: exit 1