mariadb: mytop add "clear" to PATH

[jojosch]

Motivation for this change

The following error was printed every second:

Can't exec "clear": Not a directory at /nix/store/.../bin/.mytop-wrapped line 110.
Use of uninitialized value $CLEAR in string at /nix/store/.../bin/.mytop-wrapped line 781.

Therefore the mytop output was repeatedly displayed instead of updating itself "in-place".

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[aanderse]

@jojosch when I originally "fixed" mytop and tested it my shell was at the perfect resolution that only quickly reviewing the output I didn't notice this. D'oh! Thanks for the PR. I'll test and merge this shortly.