feat(dev-generic): add ENV ZEPHYR_SDK_VERSION

Lets scripts (outside of the Zephyr bubble) locate the SDK.

PR: zmkfirmware/zmk-docker#82

Dockerfile

@@ -73,6 +73,9 @@ RUN \
   && apt-get clean \
   && rm -rf /var/lib/apt/lists/*
 
+ARG ZEPHYR_SDK_VERSION
+ENV ZEPHYR_SDK_VERSION=${ZEPHYR_SDK_VERSION}
+
 ENV DEBIAN_FRONTEND=
 
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