use a lockfile for "old" CI job

.github/workflows/spec_zero.yml

@@ -60,3 +60,9 @@ jobs:
           PR_NUM=$(gh pr create --base main --head spec_zero --title "MAINT: Update dependency specifiers" --body "Created by spec_zero [GitHub action](https://github.com/mne-tools/mne-python/actions/runs/${{ github.run_id }}). <br> <br> *Adjustments may need to be made to shims in \`mne/fixes.py\` in this or another PR. \`git grep TODO VERSION\` is a good starting point for finding potential updates.*" --label "no-changelog-entry-needed")
           echo "Opened https://github.com/mne-tools/mne-python/pull/${PR_NUM}" >> $GITHUB_STEP_SUMMARY
         if: steps.status.outputs.dirty == 'true'
+        # TODO add one or more steps here to (re)create the lockfile based on the pins
+        #      found in pyproject.toml. For now, any dependency that has a >= pin should
+        #      become an == pin, and all other pins should be unchanged. Don't forget to
+        #      pin Python version based on the `requires-python` field. Ideally, this step
+        #      should only run if the pins in pyproject.toml have changed (e.g., see how
+        #      we use "dirty" above to make the "Create PR" step conditional).

.github/workflows/tests.yml

@@ -152,7 +152,10 @@ jobs:
             python=${{ env.PYTHON_VERSION }}
             -v
         if: ${{ !startswith(matrix.kind, 'pip') }}
+        # TODO this ↑↑↑ will include matrix.kind == 'old' but we no longer want it to
         timeout-minutes: 20
+      # TODO add a step here to install uv or pixi using their provided action
+      #      it should only happen for matrix.kind == 'old'
       - run: bash ./tools/github_actions_dependencies.sh
       # Minimal commands on Linux (macOS stalls)
       - run: bash ./tools/get_minimal_commands.sh

tools/github_actions_dependencies.sh

@@ -27,6 +27,8 @@ if [ ! -z "$CONDA_ENV" ]; then
 		GROUP="test_extra"
 		EXTRAS="[hdf5]"
 	fi
+# TODO add a conditional branch here, that uses uv/pixi to recreate the "old" env
+#      from its lockfile, activate the env, and exit the bash script
 elif [[ "${MNE_CI_KIND}" == "pip" ]]; then
 	GROUP="test_extra"
 	EXTRAS="[full-pyside6]"

tools/github_actions_env_vars.sh

@@ -15,6 +15,8 @@ if [[ "$MNE_CI_KIND" == "pip"* ]]; then
     fi
 else  # conda-like
     echo "Setting conda env vars for $MNE_CI_KIND"
+    # TODO pull this ↓↓↓ conditional branch out of its parent "else" clause, and
+    #      adjust so it no longer sets a CONDA_ENV environment variable
     if [[ "$MNE_CI_KIND" == "old" ]]; then
         echo "CONDA_ENV=tools/environment_old.yml" | tee -a $GITHUB_ENV
         echo "MNE_IGNORE_WARNINGS_IN_TESTS=true" | tee -a $GITHUB_ENV