Fix fetching open-sans by removing unwanted URL encoding of rev.

Motivation for this change

Currently fetchFromGitLab call in open-sans package fails. Apparently fetchFromGitLab doesn't need rev to be pre-URL-encoded anymore. I don't know if it ever did. With current master, building open-sans results in the following error (note revision in the final URL is double-encoded):

# nix-build -A open-sans --no-link --check
checking outputs of '/nix/store/4d4jcvhgwp896g71anr9jb3kwlin1zj7-open-sans-1.11.drv'...

trying https://salsa.debian.org/api/v4/projects/fonts-team%2Ffonts-open-sans/repository/archive.tar.gz?sha=debian%252F1.11-1
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
curl: (22) The requested URL returned error: 404
error: cannot download open-sans-1.11 from any mirror
builder for '/nix/store/4d4jcvhgwp896g71anr9jb3kwlin1zj7-open-sans-1.11.drv' failed with exit code 1
error: build of '/nix/store/4d4jcvhgwp896g71anr9jb3kwlin1zj7-open-sans-1.11.drv' failed

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.