fix(xcat-core): clean mock buildroot before each build (--init), not reuse-dirty

buildrpms.pl built every package with mock -N (--no-clean), reusing the per-package
<pkg>-<target><ext> chroot across runs for flat disk. But a build aborted or killed
mid-flight leaves that chroot half-initialised with a corrupt rpmdb; the NEXT run
reused it and failed (cannot open Packages database .../usr/lib/sysimage/rpm),
producing an incomplete core (e.g. missing xCAT-test) that fails the deploy-time
completeness gate.

Re-init the buildroot (mock --init) right before building each package, after the
diskcache skip so it only runs when actually building. --init restores from mock's
root-cache tarball (cheap) so disk stays flat and builds stay fast; -N is kept on the
srpm/binary calls so they still reuse the freshly initialised root within the run.
This stabilises builds against corrupt state left by any previous failed/aborted build.

Signed-off-by: Daniel Hilst <392820+dhilst@users.noreply.github.com>

buildrpms.pl

@@ -482,6 +482,16 @@ sub buildspkgs {
 
     say "Building $diskcache";
 
+    # Clean-before-start: re-init the buildroot from mock's root cache so a corrupt or
+    # half-built chroot left behind by a PREVIOUS aborted/failed run cannot poison this
+    # build (this is what caused "cannot open Packages database ... /usr/lib/sysimage/rpm"
+    # -> missing rpm -> incomplete core). Cheap: --init restores from the cached root
+    # tarball rather than a full dnf bootstrap, and the -N below then reuses THIS freshly
+    # initialised root for both the srpm and the binary rebuild within this run. We reach
+    # here only when actually building (past the diskcache skip), so flat-disk reuse across
+    # runs is preserved -- we just guarantee a known-good starting point each time.
+    sh_retry(qq(mock -r $chroot @{[ join "  ", @opts ]} --init));
+
     sh_retry(<<"EOF");
 mock -r $chroot \\
     -N \\