[nix] locked nixpkgs for VCS

templates/chisel/nix/pkgs/vcs-fhs-env.nix

@@ -1,11 +1,28 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: 2024 Jiuyang Liu <[email protected]>
 
-{ buildFHSEnv
-, vcStaticHome
+# This is a bit dirty.
+# Since VCS are close source toolchains, we have no way to fix it for environment changes.
+# So here we have to lock the whole nixpkgs to a working version.
+#
+# For convenience, we still use the nixpkgs defined in flake to "callPackage" this derivation.
+# But the buildFHSEnv, targetPkgs is still from the locked nixpkgs.
+{ vcStaticHome
 , snpslmdLicenseFile
+, fetchFromGitHub
 }:
-buildFHSEnv {
+let
+  nixpkgsSrcs = fetchFromGitHub {
+    owner = "NixOS";
+    repo = "nixpkgs";
+    "rev" = "c374d94f1536013ca8e92341b540eba4c22f9c62";
+    "hash" = "sha256-Z/ELQhrSd7bMzTO8r7NZgi9g5emh+aRKoCdaAv5fiO0=";
+  };
+
+  # The vcs we have only support x86-64_linux
+  lockedPkgs = import nixpkgsSrcs { system = "x86_64-linux"; };
+in
+lockedPkgs.buildFHSEnv {
   name = "vcs-fhs-env";
 
   profile = ''