bash: Disable parallel building

Parallel building randomly fails because bash's Makefiles at some
point do a recursive make invocation to build a file "version.h":

  ../version.h: ../config.h ../Makefile Makefile
          -( cd ${BUILD_DIR} && ${MAKE} ${MFLAGS} version.h )

As a result, both the parent and the child can end up building
version.h at the same time.  This causes the build to fail with

  mv: cannot stat `newversion.h': No such file or directory

http://hydra.nixos.org/build/3853249

pkgs/shells/bash/default.nix

@@ -48,7 +48,9 @@ stdenv.mkDerivation rec {
     ++ stdenv.lib.optional (texinfo != null) texinfo
     ++ stdenv.lib.optional interactive readline;
 
-  enableParallelBuilding = true;
+  # Bash randomly fails to build because of a recursive invocation to
+  # build `version.h'.
+  enableParallelBuilding = false;
 
   postInstall = ''
     # Add an `sh' -> `bash' symlink.