gopls: 0.8.0 -> 0.8.1

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Description of changes

Update gopls.

This also changes the version of go used to build it to go1.18. This is
required for gopls to work with go1.18 code.

It's still backwards compatible with go1.17 code, so building with 1.18
should allow us to support all supported go versions (1.17+1.18)
correctly, while building with 1.17 does not.

Once #164320 merges, this should switch back to buildGoModule.

Upstream release notes may be found here: https://github.com/golang/tools/releases/tag/gopls%2Fv0.8.1

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.05 Release Notes (or backporting 21.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

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fyi @zowoq, this PR should be updated to buildGoModule once your PR (linked above) merges.

If your PR merges first, I'm happy to do that here.
If this one's first, you can grab it in your PR, or I can do it as a followup.