fix(ci): drop the bash-only shopt from the Linux artifact step

"Prepare Linux artifacts" ran `shopt -s nullglob`, but the runner executes
`run:` blocks with POSIX sh, where shopt does not exist. It exited 127 and
failed the step -- so build-linux never uploaded, create-release (which
needs all three build jobs) never ran, and v0.9.0 and v0.9.1 both compiled
successfully but published nothing. The last release with assets is v0.8.2.

Reproduced under busybox sh: the current block prints "shopt: not found",
passes the unmatched rpm glob through literally ("cp: can't stat
'.../bundle/rpm/*.rpm'"), and exits 127. Without nullglob an unmatched
pattern stays literal, so test each candidate with [ -e ] instead; the
same input then exits 0 with the AppImage and deb copied.

traceability-check.yml already carries this rule in two places (`case`
instead of `[[ == ]]`, a pipe instead of a here-string). Keeping the fix
POSIX rather than adding `shell: bash` follows that convention and drops
the dependency on bash being present in the builder image.

.gitea/workflows/build-release.yml

@@ -154,14 +154,19 @@ jobs:
           #
           # `if [ -f "dir/"*.ext ]` was also wrong on its own terms: with more
           # than one match `test` gets extra arguments and fails.
-          shopt -s nullglob
+          #
+          # No `shopt -s nullglob` here: the runner executes `run:` blocks with
+          # POSIX sh, where shopt does not exist -- it exited 127 and killed the
+          # step (which is why v0.9.0 and v0.9.1 built but never published).
+          # Without nullglob an unmatched pattern stays literal, so test each
+          # candidate instead. Same POSIX-only rule as traceability-check.yml.
           for bundle in \
             src-tauri/target/release/bundle/appimage/*.AppImage \
             src-tauri/target/release/bundle/deb/*.deb \
             src-tauri/target/release/bundle/rpm/*.rpm; do
+            [ -e "$bundle" ] || continue
             cp -v "$bundle" dist/linux/
           done
-          shopt -u nullglob
 
           # A release with no Linux package is a failure, not a quiet success.
           if [ -z "$(ls -A dist/linux/)" ]; then