sbcl: create version file if none exists

Fallback for when there is no such file (e.g. building from git)

The blanket recreation of this file was removed by me in
d09db9c096 but I hadn’t considered this edge case.

pkgs/development/compilers/sbcl/default.nix

@@ -146,6 +146,12 @@ stdenv.mkDerivation (self: rec {
         --replace-quiet /bin/uname "${coreutils}/bin/uname" \
         --replace-quiet /bin/sh "${stdenv.shell}"
     )
+    # Official source release tarballs will have a version.lispexpr, but if you
+    # want to override { src = ... } it might not exist. It’s required for
+    # building, so create a mock version as a backup.
+    if [[ ! -a version.lisp-expr ]]; then
+      echo '"${version}.nixos"' > version.lisp-expr
+    fi
   '';
 
   preBuild = ''