[20.09] steam: fix readonly boostrap.tar.xz

[urlordjames]

steam writes it with 444 permissions, which makes simple deletions fails, and steam will crash

(cherry picked from commit 373cb5d)

Motivation for this change

fix #121025

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.

[r-rmcgibbo]

Result of nixpkgs-review pr 121090 at a8f4cd1 run on x86_64-linux 1

6 packages built successfully:

• kodiPlugins.steam-launcher
• lutris
• steam
• steam-run
• steam-run-native
• steamcmd

[jonringer]

LGTM

was able to launch steam

[jonringer]

thanks for your first contribution to Nixpkgs @urlordjames :)