GHA: use ubuntu-slim image in 3 jobs

For small jobs using no parallelism, and which still use x64 for faster
`apt install`. x64 1-core (vs. 4), 5GB RAM (vs. 16), no Linuxbrew,
no arm64.

Refs:
https://docs.github.com/en/actions/reference/runners/github-hosted-runners
https://github.blog/changelog/2026-01-22-1-vcpu-linux-runner-now-generally-available-in-github-actions/
285cf722f0/images/ubuntu-slim/ubuntu-slim-Readme.md

Closes #20431

.github/workflows/checksrc.yml

@@ -97,7 +97,7 @@ jobs:
 
   complexity:
     name: 'complexity'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     timeout-minutes: 3
     steps:
       - name: 'install pmccabe'
@@ -116,7 +116,7 @@ jobs:
 
   xmllint:
     name: 'xmllint'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     timeout-minutes: 3
     steps:
       - name: 'install prereqs'

.github/workflows/label.yml

@@ -22,7 +22,7 @@ permissions: {}
 jobs:
   label:
     name: 'Labeler'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     permissions:
       contents: read        # To comply with https://github.com/actions/labeler documentation
       pull-requests: write  # To edit labels on PRs