github actions: Misc updates/fixes

* Install a specific Hugo version
* Remove the -u flag from updates (we don't want/care about transitive updates)
* Change name

.github/workflows/update-themes.yml

@@ -1,8 +1,10 @@
-name: update-themes
+name: Update themes
 on:
   schedule:
     - cron: "0 0 * * *"
   workflow_dispatch:
+env:
+  HUGO_VERSION: "v0.119.0"
 jobs:
   build:
     runs-on: ubuntu-latest
@@ -28,8 +30,8 @@ jobs:
         run: |
           git config --global user.email "bep@users.noreply.github.com"
           git config --global user.name "bep"
-          go install github.com/gohugoio/hugo@v0.110.0
-          hugo mod get -u
+          go install github.com/gohugoio/hugo@${{ env.HUGO_VERSION }}
+          hugo mod get
           git add .
           git commit -am "[Bot] Update themes"
           git push --force-with-lease