stdenv: Fix hardening default for pkgsMusl to reenable -pie

defaultHardeningFlags is set to enable pie for Musl, but is not
actually used because the default is never put into
NIX_HARDENING_ENABLE. That still works for cases other than Musl
only because NIX_HARDENING_ENABLE is defaulted in the binutils and
cc-wrapper setup-hook.sh scripts.

pkgs/stdenv/generic/make-derivation.nix

@@ -276,7 +276,7 @@ in rec {
           in [ "--cross-file=${crossFile}" ] ++ mesonFlags;
         } // lib.optionalAttrs (attrs.enableParallelBuilding or false) {
           enableParallelChecking = attrs.enableParallelChecking or true;
-        } // lib.optionalAttrs (hardeningDisable != [] || hardeningEnable != []) {
+        } // lib.optionalAttrs (hardeningDisable != [] || hardeningEnable != [] || stdenv.hostPlatform.isMusl) {
           NIX_HARDENING_ENABLE = enabledHardeningOptions;
         } // lib.optionalAttrs (stdenv.hostPlatform.isx86_64 && stdenv.hostPlatform ? platform.gcc.arch) {
           requiredSystemFeatures = attrs.requiredSystemFeatures or [] ++ [ "gccarch-${stdenv.hostPlatform.platform.gcc.arch}" ];