bash: change sessionVariables to attrsOf ... (#7300)

I had programs.bash.sessionVariables.CDPATH = ... defined in the
different nix files. One was taken into account and the other one
ignored.

Turns out plain "types.attrs" has the following merging strategy:

===
merge = loc: foldl' (res: def: res // def.value) { };
===

Changing it to "attrsOf", the module system warned me of the conflicting
values.

modules/programs/bash.nix

@@ -137,7 +137,7 @@ in
 
       sessionVariables = mkOption {
         default = { };
-        type = types.attrs;
+        type = with types; lazyAttrsOf (either str int);
         example = {
           MAILCHECK = 30;
         };