fix: restore this role to assume action region

Author: samrose

.github/actions/nix-install-ephemeral/action.yml

@@ -13,7 +13,7 @@ runs:
       if: ${{ inputs.push-to-cache == 'true' }}
       with:
         role-to-assume: ${{ env.DEV_AWS_ROLE }}
-        aws-region: "ap-southeast-1"
+        aws-region: "us-east-1"
         output-credentials: true
         role-duration-seconds: 7200
     - name: Setup AWS credentials for Nix