nim: cross-compilation fixes

* Import with callPackages
* Use buildPackages for building a cross-compiler
* Patch-out potential conflicts in nim.cfg
* Generate a configuration with toolchain detection
* Build with strictDeps enabled

pkgs/development/tools/misc/nimlsp/default.nix

@@ -31,7 +31,7 @@ stdenv.mkDerivation rec {
     export HOME=$TMPDIR
     nim -d:release -p:${astpatternmatching}/src -p:${jsonschema}/src \
       c --threads:on -d:nimcore -d:nimsuggest -d:debugCommunication \
-      -d:debugLogging -d:explicitSourcePath=${srcOnly nim.unwrapped} -d:tempDir=/tmp src/nimlsp
+      -d:debugLogging -d:explicitSourcePath=${srcOnly nim.passthru.nim} -d:tempDir=/tmp src/nimlsp
   '';
 
   installPhase = ''