Install libstdc++6:amd64 in the container

Installing `nodejs:amd64` in a 32-bit container to work around
actions/checkout#334 works by causing
64-bit libraries the external mapped `node20` needs (even though
`nodejs:amd64` in Debian is Node 18), as described in
actions/checkout#334 (comment).
But this installed more packages than are needed.

It turns out that, at least with this image, the only libraries the
container is missing are the 32-bit libstdc++ and its dependencies.
It is therefore sufficient to install the `libstdc++6:amd64`
package. This commit makes that change, squashing a few
investigatory steps that led to it (#5).

* See if libssl3:amd64 is all we need

* Temporarily omit even libssl3:amd64

With only `libssl3:amd64` and its dependencies, `actions/checkout`
did fail, but with a different error than before, showing a clear
error about a missing library, `libstdc++`.

    /usr/bin/docker exec  c1e55a75713e2c4fd08241fae9f4fecbe3cbb179be45174b3138390a1238090e sh -c "cat /etc/*release | grep ^ID"
    /__e/node20/bin/node: error while loading shared libraries: libstdc++.so.6: cannot open shared object file: No such file or directory

Before trying adding that, I want to check that enabling `amd64` in
`dpkg` has the expected *no effect* on the error messages, compared
to not enabling it.

* Install libstdc++6:amd64 in the container

This does not add back libssl3:amd64 yet, in case it is not needed,
though I expect that it will be needed.

.github/workflows/ci.yml

@@ -166,7 +166,7 @@ jobs:
         run: |
           dpkg --add-architecture amd64  # For 64-bit Node.js, for actions.
           apt-get update
-          apt-get install --no-install-recommends -y build-essential ca-certificates cmake curl git jq libssl-dev nodejs:amd64 pkgconf
+          apt-get install --no-install-recommends -y build-essential ca-certificates cmake curl git jq libssl-dev libstdc++6:amd64 pkgconf
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable
         with: