ci: fix Go cache perms in mx-sdk container

[webern]

Problem

The Linux (Go + C targets) CI job fails at the Go corert suite step
(run):

cd gen/test/go && MX_REPO_ROOT=/workspace go test -count=1 -v ./corert/
go: go: could not create module cache: mkdir /go: permission denied
make: *** [Makefile:355: test-go] Error 1

The DOCKER_RUN recipe runs the mx-sdk container as the caller's uid:gid
(--user 1001:1001 on the runner). That UID has no /etc/passwd entry, so
HOME defaults to /. Go then derives GOPATH=/go and
GOCACHE=/.cache/go-build, neither of which the unprivileged user can create.

The C++ jobs don't hit this because the Dockerfile already pins CCACHE_DIR
to the writable /workspace/build volume; Go had no equivalent override.

Fix

Pin GOPATH, GOCACHE, and GOMODCACHE under the writable /workspace/build
volume in the Dockerfile, exactly mirroring CCACHE_DIR. The Go module is
vendored (gen/test/go/vendor/), so no network fetch is needed.

Base

Targets musicxml-4.0 so that merging this folds the fix into PR #169.

[github-actions]

gen-quality gen/

gen-quality: 84.5 / 100   (floor 84.5, +0.0)

  structure     86.5  x0.50   [fn 90.5 / file 82.6]
  cyclomatic    88.4  x0.25
  cognitive     76.6  x0.25

  409 functions across 31 files, 7702 lines (largest file 1044)
  max cc 56  max cognitive 44  max fn loc 152

Worst offenders (top 5 per axis; full lists in score.json):
  cyclomatic gen/xsd/analyze.py:311     report                             56
  cyclomatic gen/plates/build.py:956    _validate_config_against_ir        35
  cyclomatic gen/press/context.py:145   plate_context                      34
  cyclomatic gen/__main__.py:46         _ir                                23
  cyclomatic gen/tests/test_ir.py:102   _check_references                  20
  cognitive  gen/xsd/analyze.py:311     report                             44
  cognitive  gen/ir/resolve.py:119      flat_elements                      40
  cognitive  gen/tests/test_ir.py:102   _check_references                  38
  cognitive  gen/press/context.py:145   plate_context                      37
  cognitive  gen/xsd/analyze.py:207     _sccs                              37
  size       gen/xsd/analyze.py:311     report                             152
  size       gen/press/context.py:145   plate_context                      96
  size       gen/plates/build.py:533    _value_plate                       89
  size       gen/plates/build.py:956    _validate_config_against_ir        89
  size       gen/ir/resolve.py:119      flat_elements                      78

Commit ccc0507e7edc67e9d9fa7e76297f3f831443802a.

[github-actions]

Coverage report

Core-dev coverage src/private/mx/core/

Metric	Coverage	Covered / Total
Lines	77.9%	28505 / 36590
Functions	74.4%	6354 / 8542
Branches	50.7%	22645 / 44673

API coverage src/private/mx/{api,impl,utility}/

Metric	Coverage	Covered / Total
Lines	68.7%	4734 / 6895
Functions	53.5%	1507 / 2816
Branches	40.3%	3851 / 9562

Core HTML report | API HTML report

Commit ccc0507e7edc67e9d9fa7e76297f3f831443802a.