[nix] update build-go-cache usage

[ryantm]

Why

• build-go-cache stopped working if you pass vendorHash without also passing proxyVendor:

  … while calling the 'derivationStrict' builtin
   at <nix/derivation-internal.nix>:9:12:
      8|
      9|   strict = derivationStrict drvAttrs;
       |            ^
     10|
  … while evaluating derivation 'upm-0.0.0-dirty'
   whose name attribute is located at /nix/store/xxg0317r42hi568l55dvilyn92kgdrd0-source/pkgs/stdenv/generic/make-derivation.nix:300:7
  … while evaluating attribute 'buildInputs' of derivation 'upm-0.0.0-dirty'
   at /nix/store/xxg0317r42hi568l55dvilyn92kgdrd0-source/pkgs/stdenv/generic/make-derivation.nix:347:7:
    346|       depsHostHost                = lib.elemAt (lib.elemAt dependencies 1) 0;
    347|       buildInputs                 = lib.elemAt (lib.elemAt dependencies 1) 1;
       |       ^
    348|       depsTargetTarget            = lib.elemAt (lib.elemAt dependencies 2) 0;
  (stack trace truncated; use '--show-trace' to show the full, detailed trace)
  error: cannot coerce null to a string: null

What changed

• Pin build-go-cache to the version we are using in other projects, and the version that broke things
• Pass proxyVendor = true

Test plan

• nix build worked