try unspecifying the runner

Author: exaexa

.github/workflows/main.yml

@@ -11,7 +11,6 @@ on:
 jobs:
   build:
     name: Build thesis PDFs
-    runs-on: ubuntu-latest
     container: { image: 'aergus/latex' }
 
     steps:
@@ -27,7 +26,6 @@ jobs:
 
   verify:
     name: Verify PDF/A
-    runs-on: ubuntu-latest
     needs: build
     container: { image: 'ghcr.io/mff-cuni-cz/cuni-thesis-validator' }