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>

.github/workflows/main.yml

@@ -344,7 +344,7 @@ jobs:
     steps:
     - uses: actions/checkout@v4
       if: matrix.vector.jobname != 'linux32'
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v1 # cannot be upgraded because Node.js Actions aren't supported in this container
       if: matrix.vector.jobname == 'linux32'
     - run: ci/install-docker-dependencies.sh
     - run: ci/run-build-and-tests.sh
@@ -359,7 +359,7 @@ jobs:
         path: ${{env.FAILED_TEST_ARTIFACTS}}
     - name: Upload failed tests' directories
       if: failure() && env.FAILED_TEST_ARTIFACTS != '' && matrix.vector.jobname == 'linux32'
-      uses: actions/upload-artifact@v1
+      uses: actions/upload-artifact@v1 # cannot be upgraded because Node.js Actions aren't supported in this container
       with:
         name: failed-tests-${{matrix.vector.jobname}}
         path: ${{env.FAILED_TEST_ARTIFACTS}}