refactor: Split mkdir -m … into mkdir + chmod

As recommended by ShellCheck
<https://www.shellcheck.net/wiki/SC2174>.

nixos/modules/system/activation/activation-script.nix

@@ -217,7 +217,8 @@ in
       ''
         # Various log/runtime directories.
 
-        mkdir -m 1777 -p /var/tmp
+        mkdir -p /var/tmp
+        chmod 1777 /var/tmp
 
         # Empty, immutable home directory of many system accounts.
         mkdir -p /var/empty
@@ -231,7 +232,8 @@ in
 
     system.activationScripts.usrbinenv = if config.environment.usrbinenv != null
       then ''
-        mkdir -m 0755 -p /usr/bin
+        mkdir -p /usr/bin
+        chmod 0755 /usr/bin
         ln -sfn ${config.environment.usrbinenv} /usr/bin/.env.tmp
         mv /usr/bin/.env.tmp /usr/bin/env # atomically replace /usr/bin/env
       ''
@@ -251,7 +253,8 @@ in
           if mountpoint -q "$mountPoint"; then
             local options="remount,$options"
           else
-            mkdir -m 0755 -p "$mountPoint"
+            mkdir -p "$mountPoint"
+            chmod 0755 "$mountPoint"
           fi
           mount -t "$fsType" -o "$options" "$device" "$mountPoint"
         }

nixos/modules/system/boot/binfmt.nix

@@ -317,7 +317,8 @@ in {
     environment.etc."binfmt.d/nixos.conf".source = builtins.toFile "binfmt_nixos.conf"
       (lib.concatStringsSep "\n" (lib.mapAttrsToList makeBinfmtLine config.boot.binfmt.registrations));
     system.activationScripts.binfmt = stringAfter [ "specialfs" ] ''
-      mkdir -p -m 0755 /run/binfmt
+      mkdir -p /run/binfmt
+      chmod 0755 /run/binfmt
       ${lib.concatStringsSep "\n" (lib.mapAttrsToList activationSnippet config.boot.binfmt.registrations)}
     '';
     systemd = lib.mkIf (config.boot.binfmt.registrations != {}) {