gnat*: take bootstrap gnat compiler from buildPackages

Due to overriding and use in string interpolation, our splicing magic
doesn't work and we need to do this explicitly.

pkgs/top-level/all-packages.nix

@@ -11212,7 +11212,7 @@ in
     langCC = false;
     langAda = true;
     profiledCompiler = false;
-    inherit gnatboot;
+    inherit (buildPackages) gnatboot;
   });
 
   gnat9 = wrapCC (gcc9.cc.override {
@@ -11221,7 +11221,7 @@ in
     langCC = false;
     langAda = true;
     profiledCompiler = false;
-    gnatboot = gnat6;
+    gnatboot = buildPackages.gnat6;
   });
 
   gnat10 = wrapCC (gcc10.cc.override {
@@ -11230,7 +11230,7 @@ in
     langCC = false;
     langAda = true;
     profiledCompiler = false;
-    gnatboot = gnat6;
+    gnatboot = buildPackages.gnat6;
   });
 
   gnat11 = wrapCC (gcc11.cc.override {
@@ -11239,7 +11239,7 @@ in
     langCC = false;
     langAda = true;
     profiledCompiler = false;
-    gnatboot = gnat6;
+    gnatboot = buildPackages.gnat6;
   });
 
   gnatboot = wrapCC (callPackage ../development/compilers/gnatboot { });