ci: run nightly only

Author: Berrysoft

.github/workflows/build.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ${{ matrix.setup.os }}
     strategy:
       matrix:
-        toolchain: ['nightly', 'beta', 'stable']
+        toolchain: ['nightly']
         setup:
           - os: 'ubuntu-22.04'
           - os: 'ubuntu-24.04'