ci: Fix missing checksum in AUR package

The sha512sum command hidden within a subshell
required to be executed in the repo root directory.

.github/workflows/release_nightly.yml

@@ -578,9 +578,8 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Update PKGBUILD
-        working-directory: desktop/packages/linux/aur/ruffle-nightly-bin
         run: >
-          sed -i ./PKGBUILD
+          sed -i desktop/packages/linux/aur/ruffle-nightly-bin/PKGBUILD
           -e "s/@VERSION@/${{ steps.current_time_dots.outputs.formattedTime }}/"
           -e "s/@SHA512SUM@/$(sha512sum ${{ needs.create-nightly-release.outputs.package_prefix }}-linux-x86_64.tar.gz | cut -d' ' -f1)/"