golang: Clean internal build cache after part is compiled/tested

@tomo reports that parts/golang1.17/pkg/obj/go-build takes ~ 1GB which makes it
wasteful if we want to upload result of compilation to shacache.

It turns out we can drop that particular build cache completely, because it is
used only during the build and test of Go itself and its standard library. And
when Go is installed it will be another - "user" build cache - that will be
used to maintain and reuse build artifacts. For the reference: in SlapOS that
latter "user" build cache is located inside go.work/

-> Purge internal build cache after compilation is over.

Size of parts/golang1.17 before and after hereby patch:

before: 1.6G
after:  447M

See also related discussion in nexedi/slapos!929 (comment 128379)

/reviewed-by @tomo, @jerome
/reviewed-on nexedi/slapos!1169