fix: check if user exists before adding him to kvm group

.github/actions/prepare/action.yaml

@@ -26,9 +26,11 @@ runs:
         curl -4 -L https://github.com/ccache/ccache/releases/download/v${CCACHE_VERSION}/ccache-${CCACHE_VERSION}-linux-${OS_ARCH}.tar.xz \
           | tar -xJ -C /usr/local/bin/ --strip-components=1 --no-same-owner ccache-${CCACHE_VERSION}-linux-${OS_ARCH}/ccache
         ls -la /usr/local/bin/ccache
-    - name: add user github to kvm group
+    - name: add user github to kvm group if exists
       shell: bash
       run: |
+        id -u github 2>/dev/null && {
         sudo usermod -a -G kvm github
         id github
         grep kvm /etc/group
+        } || echo "user github not found"