Have "Release" job test release/v1 branch (#61)

Instead of the v1 tag, that is. This makes it possible to push changes
to release/v1 branch and get CI test them without actually moving the
release tag which effectively publishes the updates to users.

Author: ilammy

.github/workflows/release.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: windows-latest
     steps:
       - name: Setup Developer Command Prompt
-        uses: ilammy/msvc-dev-cmd@v1
+        uses: ilammy/msvc-dev-cmd@release/v1
       - name: Check out source code
         uses: actions/checkout@v2
       - name: Compile and run some C code