ci: build the in-tree SDCC and keep it between runs

The workflow and the Dockerfile install what building SDCC needs instead
of the distribution's sdcc, and the workflow caches toolchain/ keyed on
the contents of sdcc/.

Dockerfile

@@ -3,7 +3,14 @@ FROM debian:13-slim
 RUN apt-get update && apt-get install -y \
     make \
     gcc \
-    sdcc \
+    g++ \
+    bison \
+    flex \
+    libboost-dev \
+    patch \
+    bzip2 \
+    curl \
+    ca-certificates \
     xxd \
     python3 \
     libjson-c-dev \