fix: replace echo -e heredoc with brace-block in build-variant tags steps

echo -e doesn't interpret \n in /bin/sh (dash), which is the default
shell in catthehacker/ubuntu:act-latest. This caused steps.tags.outputs.tags
to be empty, resulting in 'tag is needed when pushing to registry' from buildx.

Also fixes a secondary bug: TAGS='${TAGS}\n...' stored a literal backslash-n
rather than a real newline, which would have broken multi-tag output when
promote_latest=true.

Fix: replace with a brace block using plain echo, which produces actual newlines
and works in both sh and bash.

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

@@ -355,11 +355,13 @@ jobs:
         id: tags
         run: |
           VERSION="${{ inputs.release_tag }}"
-          TAGS="${IMAGE}:${VERSION}"
+          { echo "tags<<EOF"
+            echo "${IMAGE}:${VERSION}"
             if [ "${{ inputs.promote_latest }}" = "true" ]; then
-            TAGS="${TAGS}\n${IMAGE}:latest"
+              echo "${IMAGE}:latest"
             fi
-          echo -e "tags<<EOF\n${TAGS}\nEOF" >> "$GITHUB_OUTPUT"
+            echo "EOF"
+          } >> "$GITHUB_OUTPUT"
       - uses: docker/build-push-action@v7
         with:
           context: .
@@ -401,11 +403,13 @@ jobs:
         id: tags
         run: |
           VERSION="${{ inputs.release_tag }}"
-          TAGS="${IMAGE}:${VERSION}-omos"
+          { echo "tags<<EOF"
+            echo "${IMAGE}:${VERSION}-omos"
             if [ "${{ inputs.promote_latest }}" = "true" ]; then
-            TAGS="${TAGS}\n${IMAGE}:latest-omos"
+              echo "${IMAGE}:latest-omos"
             fi
-          echo -e "tags<<EOF\n${TAGS}\nEOF" >> "$GITHUB_OUTPUT"
+            echo "EOF"
+          } >> "$GITHUB_OUTPUT"
       - uses: docker/build-push-action@v7
         with:
           context: .
@@ -447,11 +451,13 @@ jobs:
         id: tags
         run: |
           VERSION="${{ inputs.release_tag }}"
-          TAGS="${IMAGE}:${VERSION}-with-pi"
+          { echo "tags<<EOF"
+            echo "${IMAGE}:${VERSION}-with-pi"
             if [ "${{ inputs.promote_latest }}" = "true" ]; then
-            TAGS="${TAGS}\n${IMAGE}:latest-with-pi"
+              echo "${IMAGE}:latest-with-pi"
             fi
-          echo -e "tags<<EOF\n${TAGS}\nEOF" >> "$GITHUB_OUTPUT"
+            echo "EOF"
+          } >> "$GITHUB_OUTPUT"
       - uses: docker/build-push-action@v7
         with:
           context: .
@@ -493,11 +499,13 @@ jobs:
         id: tags
         run: |
           VERSION="${{ inputs.release_tag }}"
-          TAGS="${IMAGE}:${VERSION}-omos-with-pi"
+          { echo "tags<<EOF"
+            echo "${IMAGE}:${VERSION}-omos-with-pi"
             if [ "${{ inputs.promote_latest }}" = "true" ]; then
-            TAGS="${TAGS}\n${IMAGE}:latest-omos-with-pi"
+              echo "${IMAGE}:latest-omos-with-pi"
             fi
-          echo -e "tags<<EOF\n${TAGS}\nEOF" >> "$GITHUB_OUTPUT"
+            echo "EOF"
+          } >> "$GITHUB_OUTPUT"
       - uses: docker/build-push-action@v7
         with:
           context: .