Publishing nightly builds from 4.9 release branch (#5852)

### Description
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Mention this in the PR details and ensure a patch release is considered
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Does exactly what it says on the tin.

### How has this been tested?
<!-- Tests can be added to
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<small>By submitting this pull request, I confirm that my contribution
is made under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>

---------

Co-authored-by: Siva Somayyajula <somayyas@amazon.com>

.github/workflows/nightly-build.yml

@@ -23,11 +23,11 @@ on:
   workflow_dispatch:
 
 jobs:
-  nightly-build-for-master:
+  nightly-build-for-4-9:
     if: github.repository_owner == 'dafny-lang'
     uses: ./.github/workflows/nightly-build-reusable.yml
     with:
-      ref: master
+      ref: 4.9
       publish-prerelease: true
     secrets:
       nuget_api_key: ${{ secrets.NUGET_API_KEY }}