Fix postgresql-musl dev↔lib reference cycle

The postgresql multi-output build for musl cross-compilation fails with:
  error: cycle detected in build of '...-postgresql-aarch64-unknown-linux-musl-17.7.drv'
  in the references of output 'dev' from output 'lib'

The lib output picks up references to dev through .la files (libtool
archives with baked-in -I/-L paths) and possibly .pc files. Since dev
already references lib (for -L flags in pkgconfig), this creates a cycle.

Fix by adding postFixup that:
1. Deletes .la files from lib (unnecessary for runtime, embed dev paths)
2. Strips remaining dev references from lib via remove-references-to

Author: angerman

flake.nix

@@ -171,12 +171,16 @@
              outputChecks = {};
              separateDebugInfo = false;
              disallowedReferences = [];
-             # Bump cache breaker v3: pre-built GCC on hydra with sandbox
-             # /usr/include, cleared all FailedPaths. Orphaned paths cleaned
-             # on all builders 2026-02-19.
-             preBuild = (old.preBuild or "") + ''
-               # cache breaker v3: 2026-02-19 all orphaned paths cleaned
-               true
+             # Break the dev↔lib reference cycle: dev naturally references
+             # lib (via .pc -L flags), but lib also picks up dev references
+             # through .la files and baked-in paths. Strip dev refs from lib
+             # since lib is a runtime output that doesn't need dev at runtime.
+             postFixup = (old.postFixup or "") + ''
+               # Delete .la files from lib — libtool archives embed absolute
+               # paths to dev headers/libs and are unnecessary for runtime.
+               find "$lib" -name '*.la' -delete
+               # Strip any remaining references to $dev from the lib output.
+               find "$lib" -type f -exec remove-references-to -t "$dev" {} +
              '';
            });
          });