ci: use GitHub-hosted ubuntu-latest runners

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.

Author: GeiserX

.github/workflows/main.yml

@@ -6,11 +6,7 @@ on:
     branches: [main]
 jobs:
   lint:
-    runs-on: [self-hosted, Linux, X64]
-    # Do not run untrusted fork PR code on the self-hosted runner.
-    if: >-
-      github.event_name != 'pull_request' ||
-      github.event.pull_request.head.repo.full_name == github.repository
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
       - uses: GeiserX/awesome-lint-extra@v1.1.0
@@ -19,11 +15,7 @@ jobs:
           badge_types: '["stars", "last-commit", "language", "license"]'
           check_alphabetical: 'true'
   links:
-    runs-on: [self-hosted, Linux, X64]
-    # Do not run untrusted fork PR code on the self-hosted runner.
-    if: >-
-      github.event_name != 'pull_request' ||
-      github.event.pull_request.head.repo.full_name == github.repository
+    runs-on: ubuntu-latest
     continue-on-error: true
     steps:
       - uses: actions/checkout@v6

.github/workflows/stale.yml

@@ -8,7 +8,7 @@ permissions:
 
 jobs:
   stale:
-    runs-on: [self-hosted, Linux, X64]
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/stale@v10
         with:

.github/workflows/sync-maintainers.yml

@@ -11,7 +11,7 @@ permissions:
 
 jobs:
   sync:
-    runs-on: [self-hosted, Linux, X64]
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6