abseil-cpp: allow overriding cxxStandard

individual abseil-cpp_* versions allow overriding the C++ standard
version that abseil is built with. this is very useful when the default
compiler C++ standard is different than the project's C++ standard.
accordingly, we extend this capability to the top-level abseil-cpp
wrapper alias as there's no reason to pin to an abseil LTS just for
this.

pkgs/by-name/ab/abseil-cpp/package.nix

@@ -5,4 +5,8 @@
 # library.”  Therefore, we keep packages `abseil-cpp_YYYYMM` for each
 # required LTS branch, leaving `abseil-cpp` as an alias.
 
-{ abseil-cpp_202601 }: abseil-cpp_202601
+{
+  abseil-cpp_202601,
+  cxxStandard ? null,
+}:
+abseil-cpp_202601.override { inherit cxxStandard; }