fix(nix): pin capnpc source to immutable commit, not mutable main ref

xbuild's capnpc package pinned src to shen390s/c-capnproto's `main`
branch with a hardcoded sha256, so every upstream push broke `nix
develop` with a fixed-output-derivation hash mismatch (bug-918,
recurred 4x in this worktree). Override capnpc's src to a
fetchFromGitHub pinned at an immutable commit instead.

src/nixSupport/xce.nix

@@ -35,6 +35,17 @@ stdenv.mkDerivation {
     struct2json
     (capnpc.overrideAttrs (oldAttrs: {
       separateDebugInfo = true;
+      # bug-918: upstream xbuild's capnpc package.nix pins src to the
+      # MUTABLE `main` ref of shen390s/c-capnproto with a hardcoded sha256,
+      # so it hard-breaks (fixed-output hash mismatch) every time that
+      # branch moves. Pin to an immutable commit here instead so this repo
+      # stops depending on upstream's moving target.
+      src = pkgs.fetchFromGitHub {
+        owner = "shen390s";
+        repo = "c-capnproto";
+        rev = "e45ff682c4756ac5067cbac6d0f282a8ea5f0155";
+        sha256 = "sha256-9rTlcTia+s9ARGeTi+8auGvM0RkS+wZK7aIkzecaeSk=";
+      };
     }))
     which
     findutils