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the outdated pinned version we had in `./nixpkgs.nix`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 <...>
```

flake.nix

@@ -13,9 +13,5 @@
       bscOverlay = import ./overlay.nix;
       overlays.default = self.bscOverlay;
       legacyPackages.x86_64-linux = pkgs;
-
-      hydraJobs = {
-        inherit (pkgs) bsc-ci;
-      };
     };
 }

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 = {