Expose boehmgc-nix in overlay

flake.nix

@@ -188,15 +188,7 @@
               ++ [ "-DUSE_SSH=exec" ];
           });
 
-          nix =
+          boehmgc-nix = (final.boehmgc.override {
-            let
-              officialRelease = false;
-              versionSuffix =
-                if officialRelease
-                then ""
-                else "pre${builtins.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")}_${self.shortRev or "dirty"}";
-
-              boehmgc = (final.boehmgc.override {
             enableLargeConfig = true;
           }).overrideAttrs(o: {
             patches = (o.patches or []) ++ [
@@ -207,14 +199,22 @@
             ];
           });
 
+          nix =
+            let
+              officialRelease = false;
+              versionSuffix =
+                if officialRelease
+                then ""
+                else "pre${builtins.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")}_${self.shortRev or "dirty"}";
+
             in final.callPackage ./package.nix {
               inherit
-                boehmgc
                 fileset
                 stdenv
                 versionSuffix
                 ;
               officialRelease = false;
+              boehmgc = final.boehmgc-nix;
               libgit2 = final.libgit2-nix;
               lowdown = final.lowdown-nix;
               busybox-sandbox-shell = final.busybox-sandbox-shell or final.default-busybox-sandbox-shell;