Automate updating SPM [skip ci]

Author: kishikawakatsumi

.github/workflows/spm.yml

@@ -12,10 +12,14 @@ jobs:
     steps:
       - name: Checkout to the branch
         uses: actions/checkout@v3
+      - name: Install Swift
+        uses: slashmo/[email protected]
+        with:
+          version: 5.8
       - name: Update Swift Package
         run: |
           set -ex
-
+          export TOOLCHAINS=swift
           swift package update --package-path ./
           swift package update --package-path Resources/branch_main
           swift package update --package-path Resources/branch_release-5.9