Explicitly assign --buildroot in the rpmbuild invocation
Explicitly assign --buildroot in the rpmbuild invocation

Occasionally, I see the following error messages when running a pkg_rpm target:
`
INFO: Analysed target //foo/deploy:pkgs (1 packages loaded).
INFO: Found 1 target...
INFO: From Executing genrule //foo/deploy:pkgs:
error: cannot open Packages database in /home/mmikitka/.rpmdb
`
This PR explicitly sets the --buildroot to address this error message.

Closes #5102.

PiperOrigin-RevId: 195254300

ButterflyNetwork/bazel,katre/bazel,katre/bazel,ulfjack/bazel,werkt/bazel,akira-baruah/bazel,dslomov/bazel,dslomov/bazel-windows,davidzchen/bazel,dslomov/bazel,perezd/bazel,meteorcloudy/bazel,aehlig/bazel,ulfjack/bazel,davidzchen/bazel,bazelbuild/bazel,ButterflyNetwork/bazel,bazelbuild/bazel,dslomov/bazel,werkt/bazel,dslomov/bazel-windows,dslomov/bazel,akira-baruah/bazel,aehlig/bazel,meteorcloudy/bazel,ButterflyNetwork/bazel,dropbox/bazel,twitter-forks/bazel,meteorcloudy/bazel,dropbox/bazel,ButterflyNetwork/bazel,cushon/bazel,twitter-forks/bazel,ButterflyNetwork/bazel,twitter-forks/bazel,aehlig/bazel,dslomov/bazel,dslomov/bazel-windows,ulfjack/bazel,werkt/bazel,dslomov/bazel,akira-baruah/bazel,ulfjack/bazel,katre/bazel,bazelbuild/bazel,aehlig/bazel,safarmer/bazel,dropbox/bazel,ButterflyNetwork/bazel,perezd/bazel,davidzchen/bazel,twitter-forks/bazel,twitter-forks/bazel,perezd/bazel,katre/bazel,aehlig/bazel,dropbox/bazel,bazelbuild/bazel,meteorcloudy/bazel,ulfjack/bazel,katre/bazel,bazelbuild/bazel,ulfjack/bazel,bazelbuild/bazel,davidzchen/bazel,meteorcloudy/bazel,dslomov/bazel-windows,safarmer/bazel,aehlig/bazel,safarmer/bazel,cushon/bazel,akira-baruah/bazel,cushon/bazel,werkt/bazel,werkt/bazel,twitter-forks/bazel,dslomov/bazel,cushon/bazel,davidzchen/bazel,safarmer/bazel,dropbox/bazel,cushon/bazel,perezd/bazel,meteorcloudy/bazel,dslomov/bazel-windows,safarmer/bazel,safarmer/bazel,ulfjack/bazel,aehlig/bazel,davidzchen/bazel,akira-baruah/bazel,perezd/bazel,cushon/bazel,meteorcloudy/bazel,werkt/bazel,davidzchen/bazel,katre/bazel,dropbox/bazel,twitter-forks/bazel,akira-baruah/bazel,perezd/bazel,dslomov/bazel-windows,perezd/bazel