Run 'make installcheck' again

This was failing because the check for the existence of the
'installcheck' target failed silently, so the whole phase got
skipped. It works by running 'make -n installcheck 2> /dev/null',
which however barfs with

  /nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld.gold: error: cannot open tests/plugins/plugintest.o: No such file or directory

Fixes #8004.

flake.nix

@@ -383,6 +383,7 @@
 
  doInstallCheck = finalAttrs.doCheck;
  installCheckFlags = "sysconfdir=$(out)/etc";
+ installCheckTarget = "installcheck"; # work around buggy detection in stdenv
 
  separateDebugInfo = !currentStdenv.hostPlatform.isStatic;