github: Fix main workflow by using 'ubuntu-22.04' image

The 'ubuntu-latest' image was Ubuntu 22.04 but has been Ubuntu 24.04,
and this caused the 'fontawesome5.tar.xz' package failing to install:

```
TeXLive::TLUtils::check_file_and_remove: neither checksum nor checksize available for /tmp/sZbRLtVCfX/7Rk9CYBNzM/fontawesome5.tar.xz, cannot check integrityUndefined subroutine &TeXLive::TLPDB::_post_install_package called at /usr/share/texlive/tlpkg/TeXLive/TLPDB.pm line 1665, <TMP> line 54.
```

Fix it by changing 'ubuntu-latest' to 'ubuntu-22.04'.

Issue: #23

Author: liweitianux

.github/workflows/main.yml

@@ -9,7 +9,8 @@ on:
     - master
 jobs:
   all:
-    runs-on: ubuntu-latest
+    # see https://github.com/actions/runner-images
+    runs-on: ubuntu-22.04
     steps:
     - uses: actions/checkout@v4
     - name: install deps