ci(linux32): add a note about Actions that must not be updated

ci(linux32): add a note about Actions that must not be updated

The Docker container used by the `linux32` job comes without Node.js,
and therefore the `actions/checkout` and `actions/upload-artifact`
Actions cannot be upgraded to the latest versions (because they use
Node.js).

One time too many, I accidentally tried to update them, where
`actions/checkout` at least fails immediately, but the
`actions/upload-artifact` step is only used when any test fails, and
therefore the CI run usually passes even though that Action was updated
to a version that is incompatible with the Docker container in which
this job runs.

So let's add a big fat warning, mainly for my own benefit, to avoid
running into the very same issue over and over again.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>