This does not yet include support for the //go:embed directive added
in this release.
* Makefile.am (check-runtime): Don't create check-runtime-dir.
(mostlyclean-local): Don't remove check-runtime-dir.
(check-go-tool, check-vet): Copy in go.mod and modules.txt.
(check-cgo-test, check-carchive-test): Add go.mod file.
* Makefile.in: Regenerate.
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/280172
