chronos: fix PATH setting (#12845)

Currently the $PATH bit is expanded, i.e. the ENV string becomes the
PATH of the host system, as opposed to the docker container's PATH. We
should escape the `$` to ensure we don't overwrite the full PATH

infra/experimental/chronos/prepare-ccache

@@ -18,5 +18,5 @@
 PROJECT=$1
 {
     echo "COPY ccache-cache/ /ccache/cache";
-    echo "ENV PATH=/ccache/bin:$PATH"
+    echo "ENV PATH=\"/ccache/bin:\$PATH\""
 } >> "projects/$PROJECT/Dockerfile"