shell.nix: pin bazel version

Change-Id: I34e665bf19069162a5ee671147eaf057a26e6381
Reviewed-on: https://gerrit.hackerspace.pl/c/hscloud/+/1388
Reviewed-by: q3k <q3k@hackerspace.pl>
diff --git a/shell.nix b/shell.nix
index 2cc18f2..5dd597b 100644
--- a/shell.nix
+++ b/shell.nix
@@ -42,7 +42,7 @@
 in (pkgs.buildFHSUserEnv {
   name = "hscloud-build";
   targetPkgs = pkgs: with pkgs; [
-    bazel
+    bazel_4
     postgresql
     python38
     openjdk11