ci: use GitHub-hosted ubuntu-latest runners (#24)

Public repos get free, unlimited standard GitHub-hosted runners. Move
self-hosted Linux CI jobs to ubuntu-latest: frees the home-lab servers,
runs PR code in throwaway isolated VMs (removing the fork-PR attack
surface), and costs nothing on public repos. The previous self-hosted
fork-guard is removed as it is no longer needed.

Co-authored-by: GeiserX <drumsergio@gmail.com>

Author: GeiserX

.github/workflows/desktop-release.yml

@@ -83,7 +83,7 @@ jobs:
   publish:
     needs: build
     if: needs.build.result == 'success'
-    runs-on: [self-hosted, Linux, X64]
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
 

.github/workflows/stale.yml

@@ -10,7 +10,7 @@ permissions:
 
 jobs:
   stale:
-    runs-on: [self-hosted, Linux, X64]
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/stale@v10
         with: