Fix cache issues with self-hosted runners

Disable julia-actions/cache for self-hosted runners as they persist between runs,
which causes the cache action to fail when the depot already exists.

This change:
- Adds conditional logic to detect self-hosted runners via USE_SELF_HOSTED repository variable
- Passes self-hosted=true when USE_SELF_HOSTED is set
- Disables cache when running on self-hosted runners

To use self-hosted runners, set the USE_SELF_HOSTED repository variable to 'true'.

Fixes the issue discussed here: https://github.com/SciML/ModelingToolkit.jl/actions/runs/16840335621/job/47712027510

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

.github/workflows/Tests.yml

@@ -45,4 +45,8 @@ jobs:
     with:
       julia-version: "${{ matrix.version }}"
       group: "${{ matrix.group }}"
+      # Disable cache for self-hosted runners since they persist between runs
+      # Set USE_SELF_HOSTED repository variable to 'true' when using self-hosted runners
+      self-hosted: ${{ vars.USE_SELF_HOSTED == 'true' }}
+      cache: ${{ vars.USE_SELF_HOSTED != 'true' }}
     secrets: "inherit"