macos: update Sparkle to 2.6.3

There are numerous fixes since our previous version (2.5.1) and I
believe at least one is one that users have hit where unarchiving didn't
work properly despite a properly built archive. I'm hoping this improves
that.

Besides this, there is one major security fix. I think it was low risk
to our project currently but I read through it and it is sensible to
protect against the case.

.github/workflows/release-pr.yml

@@ -29,7 +29,7 @@ jobs:
       # Setup Sparkle
       - name: Setup Sparkle
         env:
-          SPARKLE_VERSION: 2.5.1
+          SPARKLE_VERSION: 2.6.3
         run: |
           mkdir -p .action/sparkle
           cd .action/sparkle

.github/workflows/release-tip.yml

@@ -53,7 +53,7 @@ jobs:
       # Setup Sparkle
       - name: Setup Sparkle
         env:
-          SPARKLE_VERSION: 2.5.1
+          SPARKLE_VERSION: 2.6.3
         run: |
           mkdir -p .action/sparkle
           cd .action/sparkle