fix(ci): guard count-elements step against bst show failures

With bash -e active (GHA default), `BST_SHOW_OUT=$(cmd 2>&1)` exits
immediately if cmd fails, so `rc=$?` never runs and the step exits
with the bst show exit code (255 when BST can't load the element graph
e.g. a patch queue fails to apply).

Wrap the assignment with `set +e` / `set -e` so the error is
captured in `rc` and the existing graceful fallback (total=0 warning)
executes instead of killing the build job entirely.

Fixes nvidia and default build failures observed in runs:
- 28056225668 (next branch, count step exits 255)
- 28056221904 (auto/track-next-junction, validate bst show fails)

Assisted-by: Claude Sonnet 4.6 via GitHub Copilot

Author: castrojo

.github/workflows/build.yml

@@ -193,9 +193,11 @@ jobs:
         env:
           BST_FLAGS: -o x86_64_v3 true --no-interactive --config /src/buildstream-ci.conf
         run: |
+          set +e
           BST_SHOW_OUT=$(just bst show --deps all --format '%{name}' \
             ${{ matrix.element }} 2>&1)
           rc=$?
+          set -e
           if [ "$rc" -eq 0 ]; then
             TOTAL=$(echo "$BST_SHOW_OUT" | wc -l)
             echo "total=${TOTAL}" >> "$GITHUB_OUTPUT"

.github/workflows/cache-warm.yml

@@ -75,9 +75,11 @@ jobs:
         env:
           BST_FLAGS: -o x86_64_v3 true --no-interactive
         run: |
+          set +e
           BST_SHOW_OUT=$(just bst show --deps all --format '%{name}' \
             oci/bluefin.bst 2>&1)
           rc=$?
+          set -e
           if [ "$rc" -eq 0 ]; then
             TOTAL=$(echo "$BST_SHOW_OUT" | wc -l)
             echo "total=${TOTAL}" >> "$GITHUB_OUTPUT"