For example one could want to build the software with race detector,
e.g.

	[gowork]
	...
	buildflags = -race

or if you want to build a plugin for fluent-bit

	[gowork]
	...
	install	   = your.package.import.path
	buildflags = -buildmode=c-shared

and then your package will be installed as shared library into

	${gowork:directory}/pkg/`go env GOOS`_`go env GOARCH`/<your.package.import.path>.a

One could pass that location explicitly to `fluentd-bit -e ...` or
symlink to it from some .so from global directory of fluent-bit plugins.

Maybe it is not very good to use `go install` for building plugins
(instead of `go build -o ...`) but we anyway need to have ability to
specify buildflags as a generic feature, hence this patch.

/reviewed-on nexedi/slapos!274