fix zero-gravity doubled-v: add VERSION normalization to extraction RUN step

The previous commit (7cbd0532) added VERSION normalization to the DOWNLOAD
RUN step but forgot to add it to the EXTRACTION RUN step. Each Dockerfile
RUN is a fresh shell, so the extraction step fails with 'VERSION: unbound
variable' when it tries to use ${VERSION} in the SRC variable.

Fix: add VERSION="${ZERO_GRAVITY_VERSION#v}"; to the extraction RUN step
(line 73) so it defines VERSION before using it.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

zero-gravity/zerog.Dockerfile

@@ -70,6 +70,7 @@ RUN set -eu; \
 # version, nor the layout. Assert on the binaries here so the NEXT upstream reshuffle
 # reports itself at the point of failure instead.
 RUN set -eu; \
+    VERSION="${ZERO_GRAVITY_VERSION#v}"; \
     SRC="/tmp/${ZERO_GRAVITY_CHAIN_SPEC}-v${VERSION}"; \
     tar -xzf "${SRC}.tar.gz" -C /tmp; \
     mkdir -p /0g; \