[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