Update the name of the bazel target to build the nightly tarball (#7080)

The target was renamed in `82fad290285baf9763132a13b1f73de1e7919074`
from `//toolchain/install:carbon_toolchain_tar_gz_rule` to
`//toolchain/install:carbon_toolchain_tar_gz`

.github/workflows/nightly_release.yaml

@@ -93,7 +93,7 @@ jobs:
             test -c opt --stamp --remote_download_toplevel \
             --pre_release=nightly --nightly_date=${{ env.nightly_date }} \
             //toolchain \
-            //toolchain/install:carbon_toolchain_tar_gz_rule \
+            //toolchain/install:carbon_toolchain_tar_gz \
             //toolchain/install:carbon_toolchain_tar_gz_test
 
       - name: Extract the release version