fix(nix): Calculate MD5 sum of config layer correctly

The MD5 sum is used for verifying contents in the layer cache before
accidentally re-uploading, but the syntax of the hash invocation was
incorrect leading to a cache-bust on the manifest layer on every
single build (even for identical images).

tools/nixery/build-registry-image.nix

@@ -208,7 +208,7 @@ let
   } ''
     size=$(wc -c ${configJson} | cut -d ' ' -f1)
     sha256=$(sha256sum ${configJson} | cut -d ' ' -f1)
-    md5=$(openssl dgst -md5 -binary $layerPath | openssl enc -base64)
+    md5=$(openssl dgst -md5 -binary ${configJson} | openssl enc -base64)
     jq -n -c --arg size $size --arg sha256 $sha256 --arg md5 $md5 \
       '{ size: ($size | tonumber), sha256: $sha256, md5: $md5 }' \
       >> $out