CI: Fix some issues pointed out by zizmor

[felixfontein]

Fix some issues in GHA workflows pointed out by zizmor (https://github.com/woodruffw/zizmor).

Some more issues are pointed out:

error[excessive-permissions]: overly broad workflow or job-level permissions
  --> /home/felix/projects/code/github-cloned/antsibull-nightly-builds/.github/workflows/build-dumb-pypi.yml:38:1
   |
38 | / permissions:
39 | |   contents: read
40 | |   pages: write
41 | |   id-token: write
   | |_________________^ pages: write is overly broad at the workflow level
   |
   = note: audit confidence → High

error[excessive-permissions]: overly broad workflow or job-level permissions
  --> /home/felix/projects/code/github-cloned/antsibull-nightly-builds/.github/workflows/build-dumb-pypi.yml:38:1
   |
38 | / permissions:
39 | |   contents: read
40 | |   pages: write
41 | |   id-token: write
   | |_________________^ id-token: write is overly broad at the workflow level
   |
   = note: audit confidence → High

If someone else wants to work on them, feel free!