tests: isolate temporary module commands from GO111MODULE

[HNO3Miracle]

The generator round-trip tests intentionally create temporary Go modules, but their go run subprocesses inherit GO111MODULE=off from the caller. Explicitly enable module mode for those subprocesses, alongside the existing GOWORK=off and GOFLAGS=-mod=mod, so the test behavior matches its temporary-module setup.

Testing:

• go test ./generator/golang
• Built the package test binary in module mode, then ran both round-trip tests with the binary environment set to GO111MODULE=off; both pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.78%. Comparing base (0837c9b) to head (9c07973).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #610   +/-   ##
=======================================
  Coverage   99.78%   99.78%           
=======================================
  Files         283      283           
  Lines       34456    34456           
=======================================
  Hits        34382    34382           
  Misses         46       46           
  Partials       28       28           

Flag	Coverage Δ
unittests	99.78% <ø> (ø)

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