GHA/checksrc: switch xmllint job to Linux (from macOS)

macOS was chosen because xmllint comes preinstalled, saving the prereq
install step. But, macOS's xmllint jobs sometimes doesn't finish in 1m
(instead of under 1 second) and gets cancelled, causing flaky failures.
Go with Linux and an install phase (of 15s) instead.

Examples:
https://github.com/curl/curl/actions/runs/19558021722/job/56004334495

Closes #19634

.github/workflows/checksrc.yml

@@ -120,9 +120,17 @@ jobs:
 
   xmllint:
     name: 'xmllint'
-    runs-on: macos-latest
+    runs-on: ubuntu-latest
     timeout-minutes: 1
     steps:
+      - name: 'install prereqs'
+        run: |
+          sudo rm -f /etc/apt/sources.list.d/microsoft-prod.list
+          sudo apt-get -o Dpkg::Use-Pty=0 update
+          sudo rm -f /var/lib/man-db/auto-update
+          sudo apt-get -o Dpkg::Use-Pty=0 install \
+            libxml2-utils
+
       - uses: actions/checkout@1af3b93b6815bc44a9784bd300feb67ff0d1eeb3 # v6.0.0
         with:
           persist-credentials: false