chore: move bash completion into its own part

snap/snapcraft.yaml

@@ -127,10 +127,6 @@ parts:
       uv export --no-dev --no-emit-workspace --no-emit-package pywin32 --output-file uv-requirements.txt
       ${SNAP}/libexec/snapcraft/craftctl default
 
-      PYTHONPATH=$CRAFT_PART_INSTALL/lib/python3.12/site-packages \
-        python3 -m craft_cli.completion $CRAFT_PROJECT_NAME rockcraft.cli:get_app_info \
-        > $CRAFT_PART_INSTALL/completion.sh
-
       version="$("${CRAFT_STAGE}/usr/bin/python3" -c "import rockcraft;print(rockcraft.__version__)")"
       ${SNAP}/libexec/snapcraft/craftctl set version="$version"
       [ -n "$(echo $version | grep "post")" ] && grade=devel || grade=stable
@@ -141,6 +137,15 @@ parts:
       - rockcraft-libs
       - libgit2
 
+  bash-completion:
+    after: [rockcraft]
+    plugin: nil
+    build-environment:
+      - PYTHONPATH: $CRAFT_PART_INSTALL/lib/python3.12/site-packages
+    override-build: |
+      python3 -m craft_cli.completion $CRAFT_PROJECT_NAME rockcraft.cli:get_app_info \
+        > $CRAFT_PART_INSTALL/completion.sh
+
   umoci:
     plugin: make
     source: https://github.com/opencontainers/umoci.git