Bring things to new compile-r7rs

dockerfiles/Dockerfile.snow-chibi-install-test

@@ -1,4 +1,5 @@
 ARG SCHEME=chibi
+ARG IMAGE=chibi:head
 FROM schemers/chibi:head AS build
 RUN apt-get update && apt-get install -y \
     ca-certificates \
@@ -11,7 +12,8 @@ RUN git clone https://gitea.scheme.org/Retropikzel/compile-r7rs.git --depth=1
 RUN cd compile-r7rs --branch=retropikzel-dependency-fixes && make
 
 ARG SCHEME=chibi
-FROM schemers/${SCHEME}:head
+ARG IMAGE=chibi:head
+FROM schemers/${IMAGE}
 RUN apt-get update && apt-get install -y \
     build-essential \
     git \