Add scratch-rpm makefile target

[nmeyerhans]

Issue #, if available: n/a

Description of changes:

I've found it convenient when developing this package to be able to quickly generate an rpm based on an unreleased branch. So, this change a scratch-rpm makefile target that does just this.

[ec2-user@ip-10-0-1-138 amazon-ec2-net-utils]$ make scratch-rpm
tar czf ../amazon-ec2-net-utils-2.3.0-5-g3d64d70.tar.gz --exclude=.git --transform 's,^./,./amazon-ec2-net-utils-2.3.0-5-g3d64d70/,' .
rpmbuild -D "_sourcedir /home/ec2-user/amazon-ec2-net-utils/.." -D "_source_version_suffix -5-g3d64d70" \
         -D "_rpm_version_suffix .5.g3d64d70" -bb amazon-ec2-net-utils.spec
...
Wrote: /home/ec2-user/rpmbuild/RPMS/noarch/amazon-ec2-net-utils-2.3.0.5.g3d64d70-1.amzn2023.noarch.rpm
Executing(%clean): /bin/sh -e /var/tmp/rpm-tmp.IMmBGI
+ umask 022
+ cd /home/ec2-user/rpmbuild/BUILD
+ cd amazon-ec2-net-utils-2.3.0-5-g3d64d70
+ /usr/bin/rm -rf /home/ec2-user/rpmbuild/BUILDROOT/amazon-ec2-net-utils-2.3.0.5.g3d64d70-1.amzn2023.x86_64
+ RPM_EC=0
++ jobs -p
+ exit 0

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