action warns if run in matrix builds

[maxheld83]

currently, when the action is run inside a matrix build, it emits this warning:

[DeterminateCI / build (x86_64-linux, ubuntu-latest)](https://github.com/dataheld/NixPlate/actions/runs/13092039423/job/36529839793#step:5:15)
Magic Nix Cache is already running, this workflow job is in noop mode. Is the Magic Nix Cache in the workflow twice?

I can find this error message here:

flakehub-cache-action/src/index.ts

Line 28 in 146f476

"FlakeHub Cache Action is already running, this workflow job is in noop mode. Is the Magic Nix Cache in the workflow twice?";