tests/tools/Makefile: simplify
tests/tools/Makefile: simplify

1. Remove GO111MODULE=on and -mod=vendor. Those are not required
   (default) since go 1.16 (or earlier), and go 1.16 is currently
   the oldest supported release.

2. Remove go-build define:
   - cd `pwd` does not make any sense;
   - @echo > /dev/null masks any errors during tools building;
   - $(shell basename ...) could be replaced by $(notdir ...);
   - the above is not needed since we already have what we need
     as a target ($@);
   - finally, this is problematic for e.g. cpuguy83/go-md2man/v2
     (as it builds the binary named v2).

   So, just use $(GO_BUILD) -o $@ path/to/package

3. GO_BUILD does not need to be exported.

4. Mark $(BUILDDIR) target as phony -- a mere existence of the directory
   should not cause make to skip building the binaries.

K.I.S.S!

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

projectatomic/buildah,projectatomic/buildah,rhatdan/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,rhatdan/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,projectatomic/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,rhatdan/buildah