Improve package installation step in CI.

XBPS doesn't update itself by default.

.github/workflows/ci.yaml

@@ -8,7 +8,9 @@ jobs:
          - name: Checkout
            uses: actions/checkout@v1
          - name: Install packages
-           run: xbps-install -Sy gcc make bearssl-devel
+           run: |
+            xbps-install -Syu || xbps-install -yu xbps
+            xbps-install -y gcc make bearssl-devel
          - name: Build and run tests
            run: ci/run.sh
    clang-glibc:
@@ -16,7 +18,9 @@ jobs:
       container: voidlinux/voidlinux
       steps:
          - uses: actions/checkout@v1
-         - run: xbps-install -Sy clang make bearssl-devel
+         - run: |
+            xbps-install -Syu || xbps-install -yu xbps
+            xbps-install -y clang make bearssl-devel
          - run: CC=clang ci/run.sh
    clang-musl-meson:
       runs-on: ubuntu-latest