Replace deprecated actions-rs/toolchain usage in build CI workflow

We missed another usage of the actions-rs/toolchain GitHub Action inside
the CI workflow. Replace it with dtolnay/rust-toolchain as we did
everywhere else earlier.

.github/workflows/build.yml

@@ -14,11 +14,8 @@ jobs:
       - uses: actions/checkout@v4
       - name: Install dependencies
         run: sudo apt-get install --yes --no-install-recommends musl-tools
-      - uses: actions-rs/toolchain@v1
+      - uses: dtolnay/rust-toolchain@stable
         with:
-          toolchain: stable
-          profile: minimal
-          override: true
           target: x86_64-unknown-linux-musl
       - run: |
           cargo install --bin=btrfs-backup --path=. --root=. --target x86_64-unknown-linux-musl