Rename subctl binary to subctl

Currently while building a subctl is binary is named as
`subctl-$version-$arch`. This PR does following changes while creating a
tar of the binary:

1. Rename the binary to just `subctl`. This is one of the requirements
   of Krew.
2. Remove the redundant child directory so that on untar'ing the binary
   is at `subctl-$version-$arch/` path instead of
   `subctl-$version-$arch/subctl-$version`

Epic: submariner-io/enhancements#182

Signed-off-by: Janki Chhatbar <[email protected]>

Makefile

@@ -92,11 +92,11 @@ cmd/bin/subctl: cmd/bin/subctl-$(VERSION)-$(GOOS)-$(GOARCH)$(GOEXE)
 
 dist/subctl-%.tar.xz: cmd/bin/subctl-%
 	mkdir -p dist
-	tar -cJf $@ --transform "s/^cmd.bin/subctl-$(VERSION)/" $<
+	tar -cJf $@ --transform "s|^cmd/bin/subctl-[^/]*|subctl|" $<
 
 dist/subctl-%.tar.gz: cmd/bin/subctl-%
 	mkdir -p dist
-	tar -czf $@ --transform "s/^cmd.bin/subctl-$(VERSION)/" $<
+	tar -czf $@ --transform "s|^cmd/bin/subctl-[^/]*|subctl|" $<
 
 dist/subctl-checksums.txt: $(CROSS_TARBALLS)
 	cd $(@D) && sha256sum $(^F) >> $(@F)