RPM sandbox directory empty since rpmbuild 4.20.0

After updating system rpmbuild to 4.20.0 building RPM packages fail.

OS: OpenSuse Tumbleweed.

Building rules_pkg project fails with:

```
Starting local Bazel server (8.1.0) and connecting to it...
WARNING: WORKSPACE support will be removed in Bazel 9 (late 2025), please migrate to Bzlmod, see https://bazel.build/external/migration.
DEBUG: /home/toni/code/gh/rules_pkg/pkg/mappings.bzl:307:34: same source mapped to different locations an_executable an_executable.runfiles/_main/tests/an_executable
INFO: From MakeRpm tests/rpm/test_rpm_bzip2-1.1.1-2222.noarch.rpm:
Error calling rpmbuild:
Building target platforms: noarch
Building for target noarch
Executing(%mkbuilddir): /usr/bin/bash -e /tmp/tmpgpatiq6v/TMP/rpm-tmp.UTgJ0s
+ umask 022
+ cd /tmp/tmpgpatiq6v/BUILD/test_rpm_bzip2-1.1.1-build
+ test -d /tmp/tmpgpatiq6v/BUILD/test_rpm_bzip2-1.1.1-build
+ /usr/bin/chmod -Rf a+rX,u+w,g-w,o-w /tmp/tmpgpatiq6v/BUILD/test_rpm_bzip2-1.1.1-build
+ /usr/bin/rm -rf /tmp/tmpgpatiq6v/BUILD/test_rpm_bzip2-1.1.1-build
+ /usr/bin/mkdir -p /tmp/tmpgpatiq6v/BUILD/test_rpm_bzip2-1.1.1-build
+ /usr/bin/mkdir -p /tmp/tmpgpatiq6v/BUILD/test_rpm_bzip2-1.1.1-build/SPECPARTS
+ RPM_EC=0
++ jobs -p
+ exit 0
Executing(%install): /usr/bin/bash -e /tmp/tmpgpatiq6v/TMP/rpm-tmp.xnb04w
+ umask 022
+ cd /tmp/tmpgpatiq6v/BUILD/test_rpm_bzip2-1.1.1-build
+ /usr/bin/rm -rf /tmp/tmpgpatiq6v/BUILDROOT
+ /usr/bin/mkdir -p /tmp/tmpgpatiq6v
+ /usr/bin/mkdir /tmp/tmpgpatiq6v/BUILDROOT
++ dirname /test/a.ar
+ install -d /tmp/tmpgpatiq6v/BUILDROOT//test
+ cp tests/testdata/a.ar /tmp/tmpgpatiq6v/BUILDROOT//test/a.ar
cp: cannot stat 'tests/testdata/a.ar': No such file or directory
error: Bad exit status from /tmp/tmpgpatiq6v/TMP/rpm-tmp.xnb04w (%install)

RPM build errors:
Bad exit status from /tmp/tmpgpatiq6v/TMP/rpm-tmp.xnb04w (%install)

No RPM file created.
```