workflow: use github.ref_type directly in promote/update-description if-conditions

Gitea Actions evaluates 'env.PROMOTE_LATEST' as empty in YAML 'if:'
contexts even though the same env var substitutes correctly in
shell run: blocks. Result: on v1.15.0/v1.15.0b tag pushes, the
build-variant-* jobs correctly pushed latest-* aliases (shell context),
but promote-base-latest and update-description got skipped (YAML
context), so the Hub README description wasn't refreshed.

Switch to evaluating github.ref_type directly in the if-conditions —
matches the production-trigger semantics and avoids the env-var
indirection that gitea evaluates inconsistently.

.gitea/workflows/docker-publish-split.yml

@@ -526,7 +526,7 @@ jobs:
       - build-variant-omos
       - build-variant-with-pi
       - build-variant-omos-with-pi
-    if: env.PROMOTE_LATEST == 'true'
+    if: ${{ github.ref_type == 'tag' || inputs.promote_latest == 'true' }}
     runs-on: ubuntu-latest
     container:
       image: catthehacker/ubuntu:act-latest
@@ -550,7 +550,7 @@ jobs:
       - build-variant-omos
       - build-variant-with-pi
       - build-variant-omos-with-pi
-    if: env.PROMOTE_LATEST == 'true'
+    if: ${{ github.ref_type == 'tag' || inputs.promote_latest == 'true' }}
     runs-on: ubuntu-latest
     container:
       image: catthehacker/ubuntu:act-latest