Only patch /android-runner if it exists

So the `cross` Dockerfile can continue to be used for both Android
and non-Android targets.

Author: EliahKagan

etc/docker/Dockerfile.test-cross

@@ -12,5 +12,7 @@ RUN apt-get update && \
     apt-get update && \
     apt-get install --no-install-recommends -y git jq && \
     rm -rf /var/lib/apt/lists/* && \
-    sed -i.orig 's/^export LD_PRELOAD=/test "${NO_PRELOAD_CXX:-0}" != 0 || &/' /android-runner && \
+    to_patch=/android-runner && \
+    patcher='s/^export LD_PRELOAD=/test "${NO_PRELOAD_CXX:-0}" != 0 || &/' && \
+    if test -f "$to_patch"; then sed -i.orig "$patcher" -- "$to_patch"; fi && \
     git config --system gitoxide.imaginary.arbitraryVariable arbitraryValue