Fix building with GCC 9

Nixpkgs on aarch64-linux is currently stuck on GCC 9
(https://github.com/NixOS/nixpkgs/issues/208412) and using gcc11Stdenv
doesn't work either.

So use c++2a instead of c++20 for now. Unfortunately this means we
can't use some C++20 features for now (like std::span).

src/libexpr/nix-expr.pc.in

@@ -7,4 +7,4 @@ Description: Nix Package Manager
 Version: @PACKAGE_VERSION@
 Requires: nix-store bdw-gc
 Libs: -L${libdir} -lnixexpr
-Cflags: -I${includedir}/nix -std=c++20
+Cflags: -I${includedir}/nix -std=c++2a