fix: environment handling windows (host mode) (#1732)

* fix: environment handling windows (host mode)

* fixup

* fixup

* add more tests

* fixup

* fix setenv

* fixes

* [skip ci] Apply suggestions from code review

Co-authored-by: Jason Song <i@wolfogre.com>

* Update side effects

---------

Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

act/runner/testdata/windows-add-env/action.yml

@@ -0,0 +1,7 @@
+runs:
+  using: composite
+  steps:
+  - run: |
+      echo $env:GITHUB_ENV
+      echo "kEy=n/a" > $env:GITHUB_ENV
+    shell: pwsh

act/runner/testdata/windows-add-env/push.yml

@@ -25,3 +25,20 @@ jobs:
           echo "Unexpected value for `$env:key2: $env:key2"
           exit 1
         }
+    - run: |
+        echo $env:GITHUB_ENV
+        echo "KEY=test" > $env:GITHUB_ENV
+        echo "Key=expected" > $env:GITHUB_ENV
+    - name: Assert GITHUB_ENV is merged case insensitive
+      run: exit 1
+      if: env.KEY != 'expected' || env.Key != 'expected' || env.key != 'expected'
+    - name: Assert step env is merged case insensitive
+      run: exit 1
+      if: env.KEY != 'n/a' || env.Key != 'n/a' || env.key != 'n/a'
+      env:
+        KeY: 'n/a'
+    - uses: actions/checkout@v3
+    - uses: ./windows-add-env
+    - name: Assert composite env is merged case insensitive
+      run: exit 1
+      if: env.KEY != 'n/a' || env.Key != 'n/a' || env.key != 'n/a'