Fix nightly docs push: don't use nonexistent 5.4 image (#78730)

Fixes #78687

Pull Request resolved: https://github.com/pytorch/pytorch/pull/78730
Approved by: https://github.com/kit1980, https://github.com/atalman, https://github.com/malfet
diff --git a/.github/workflows/nightly.yml b/.github/workflows/nightly.yml
index 3322b20..fdccfaf 100644
--- a/.github/workflows/nightly.yml
+++ b/.github/workflows/nightly.yml
@@ -18,15 +18,15 @@
     name: docs build
     uses: ./.github/workflows/_linux-build.yml
     with:
-      build-environment: linux-xenial-py3.7-gcc5.4
-      docker-image-name: pytorch-linux-xenial-py3.7-gcc5.4
+      build-environment: linux-focal-py3.7-gcc7
+      docker-image-name: pytorch-linux-focal-py3.7-gcc7
 
   docs-push:
     name: docs push
     uses: ./.github/workflows/_docs.yml
     needs: docs-build
     with:
-      build-environment: linux-xenial-py3.7-gcc5.4
+      build-environment: linux-focal-py3.7-gcc7
       docker-image: ${{ needs.docs-build.outputs.docker-image }}
       push: true
     secrets: