feat: upgrade the action to installer v12

[tonyredondo]

Summary

• upgrade the action from the v11 installer script to v12 and update the pinned checksum
• add an optional go-module-dir input and pass it through as DD_CIVISIBILITY_GO_MODULE_DIR
• document the new Go multi-module configuration and ignore .DS_Store
• add a Go CI matrix that exercises the v12 installer behavior in GitHub Actions

Go CI Matrix

The workflow now covers these Go scenarios in CI:

• root module, project go 1.24
• root module, project go 1.25
• root module, project go 1.22
• root module, runner below the supported minimum for the selected flow
• single nested module, project go 1.24, auto-detected
• single nested module, project go 1.25, auto-detected
• multiple Go modules without go-module-dir
• multiple Go modules with a valid go-module-dir
• invalid go-module-dir pointing to a missing directory

The Go jobs run go test -v ./... with DD_TRACE_DEBUG=true so the workflow logs include both verbose test output and tracer debug output.

Bug Fix Coverage

This PR now covers the regression reported in DataDog/orchestrion#806.

That issue was a fresh Go 1.24 module with no pre-existing orchestrion or dd-trace-go dependency, where the install flow drifted to an incompatible tracer line and failed because orchestrion/all/v2 v2.7.0 requires Go 1.25.

The root go1.24 success CI job exercises that same shape of project from scratch and verifies that the action completes successfully and the instrumented test run still passes on Go 1.24. There is also equivalent coverage for a single nested Go 1.24 module.

Verification

• parsed action.yml successfully after the metadata changes
• verified the live install_test_visibility_v12.sh script matches the pinned SHA-256 checksum
• confirmed the live v12 installer reads DD_CIVISIBILITY_GO_MODULE_DIR
• ran the live installer against a temporary multi-module Go workspace:
  • without the variable, Go instrumentation is skipped with guidance to set it
  • with the variable set, the installer emits GOFLAGS and DD_TRACER_VERSION_GO
• ran the PR workflow in GitHub Actions and confirmed the full matrix passes on run 23556166608

Notes

• the runner-below-minimum matrix row currently reflects the live behavior of the v12 installer as exercised through this action: it fails after partially mutating the Go module files, rather than skipping cleanly