Use nixpkgs from flake.lock and support attrs when importing bscpkgs

This makes `nix-build` and friends use the current flake lock instead of
the outdated pinned version we had in `./nixpkgs.nix`

With this, `nix-build -A ovni` and `nix build .#ovni` should produce the
same result.

This will fail if the flake nixpkgs input does not come from NixOS/nixpkgs.
We could use edolstra/flake-compat instead, but it's overkill imho.

Additionally, I made default.nix behave like nixpkgs, so that we can
import bscpkgs à la nixpkgs (Apply overlays and other options that nixpkgs
accepts):

```nix
let pkgs = import bscpkgs { inherit system; }; in <...>
```

default.nix

@@ -1,11 +1,19 @@
 let
   bscOverlay = import ./overlay.nix;
 
-  # Pin the nixpkgs
-  nixpkgsPath = import ./nixpkgs.nix;
-
-  pkgs = import nixpkgsPath {
-    overlays = [ bscOverlay ];
+  # read flake.lock and determine revision from there
+  lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+  inherit (lock.nodes.nixpkgs.locked) rev narHash;
+  fetchedNixpkgs = builtins.fetchTarball {
+    url = "https://github.com/NixOS/nixpkgs/archive/${rev}.tar.gz";
+    sha256 = narHash;
   };
-
-in pkgs
+in
+{ overlays ? [ ]
+, nixpkgs ? fetchedNixpkgs
+, ...
+}@attrs:
+import nixpkgs (
+  (builtins.removeAttrs attrs [ "overlays" "nixpkgs" ]) //
+  { overlays = [ bscOverlay ] ++ overlays; }
+)

nixpkgs.nix

@@ -1,9 +0,0 @@
-let
-  commit = "e4ad989506ec7d71f7302cc3067abd82730a4beb";
-in builtins.fetchTarball {
-    # Descriptive name to make the store path easier to identify
-    name = "nixpkgs-${commit}";
-    url = "https://github.com/nixos/nixpkgs/archive/${commit}.tar.gz";
-    # Hash obtained using `nix-prefetch-url --unpack <url>`
-    sha256 = "sha256-de9KYi8rSJpqvBfNwscWdalIJXPo8NjdIZcEJum1mH0=";
-}

pkgs/llvm-ompss2/default.nix

@@ -9,12 +9,14 @@
 , ompss2rt ? null
 }:
 
+with lib;
+
 let
   usingNodesAndOmpv = (openmp.pname == "openmp-v" && ompss2rt.pname == "nodes");
   sameNosv = openmp.nosv == ompss2rt.nosv;
 in
 
-assert lib.assertMsg (usingNodesAndOmpv -> sameNosv) "OpenMP-V and NODES must share the same nOS-V";
+assert assertMsg (usingNodesAndOmpv -> sameNosv) "OpenMP-V and NODES must share the same nOS-V";
 
 let
   homevar = if ompss2rt.pname == "nanos6" then "NANOS6_HOME" else "NODES_HOME";
@@ -34,14 +36,15 @@ let
   targetConfig = stdenv.targetPlatform.config;
   inherit gcc;
   cc = clangOmpss2Unwrapped;
+  gccVersion = with versions; let v = gcc.version; in concatStringsSep "." [(major v) (minor v) (patch v)];
 in wrapCCWith {
   inherit cc bintools;
   # extraPackages adds packages to depsTargetTargetPropagated
-  extraPackages = lib.optional (openmp != null) openmp;
+  extraPackages = optional (openmp != null) openmp;
   extraBuildCommands = ''
     echo "-target ${targetConfig}" >> $out/nix-support/cc-cflags
-    echo "-B${gcc.cc}/lib/gcc/${targetConfig}/${gcc.version}" >> $out/nix-support/cc-cflags
-    echo "-L${gcc.cc}/lib/gcc/${targetConfig}/${gcc.version}" >> $out/nix-support/cc-ldflags
+    echo "-B${gcc.cc}/lib/gcc/${targetConfig}/${gccVersion}" >> $out/nix-support/cc-cflags
+    echo "-L${gcc.cc}/lib/gcc/${targetConfig}/${gccVersion}" >> $out/nix-support/cc-ldflags
     echo "-L${gcc.cc.lib}/lib" >> $out/nix-support/cc-ldflags
 
     for dir in ${gcc.cc}/include/c++/*; do
@@ -55,12 +58,12 @@ in wrapCCWith {
 
     wrap clang++  $wrapper $ccPath/clang++
 
-  '' + lib.optionalString (openmp != null) ''
+  '' + optionalString (openmp != null) ''
     echo "export OPENMP_RUNTIME=${ompname}" >> $out/nix-support/cc-wrapper-hook
-  '' + lib.optionalString (ompss2rt != null) ''
+  '' + optionalString (ompss2rt != null) ''
     echo "export OMPSS2_RUNTIME=${rtname}" >> $out/nix-support/cc-wrapper-hook
     echo "export ${homevar}=${ompss2rt}"   >> $out/nix-support/cc-wrapper-hook
-  '' + lib.optionalString (ompss2rt != null && ompss2rt.pname == "nodes") ''
+  '' + optionalString (ompss2rt != null && ompss2rt.pname == "nodes") ''
     echo "export NOSV_HOME=${ompss2rt.nosv}" >> $out/nix-support/cc-wrapper-hook
   '';
 }

pkgs/llvm-ompss2/openmp.nix

@@ -68,6 +68,7 @@ stdenv.mkDerivation rec {
     rm -f $out/lib/libiomp*
   '' + lib.optionalString enableNosv ''
     rm -f $out/lib/libomp.*
+    rm -f $out/libllvmrt/libomp.*
   '';
 
   passthru = {