fix: remove obsolete noswissmap GOEXPERIMENT override

[ping-ke]

Summary

Remove the unconditional GOEXPERIMENT=noswissmap override from the Go build and test workflow.

Problem

The project declares Go 1.21 compatibility, but its build scripts always set:

GOEXPERIMENT=noswissmap

Go versions that do not recognize the swissmap experiment fail before building:

go: unknown GOEXPERIMENT swissmap
make: *** [Makefile:19: build/.built] Error 2

The override was added as a temporary workaround for a Go 1.24 conflict involving an external dependency. That dependency has since been removed, so the workaround is no longer needed.

Changes

• Remove the global GOEXPERIMENT=noswissmap export from Makefile.
• Run go build and go vet normally from build.sh.
• Run go test normally from the Makefile and test runner.

Behavior Changes

Go build and test commands now use the selected Go toolchain's default experiment settings. This restores compatibility with the Go version declared in go.mod and avoids the unknown GOEXPERIMENT swissmap failure.

No Go source code, dependencies, or public APIs are changed.

Validation

make rebuild 
make test-all

[qzhodl]

Does this mean that before this change, make rebuild would fail with newer Go versions, such as Go 1.24 or later? I’m wondering why I didn’t run into this issue on my side before.