Fix failing API unit test on rawhide (RhBug:2261066)

For: https://bugzilla.redhat.com/show_bug.cgi?id=2261066

Fixes:
1: ERROR: test_do_transaction (tests.api.test_dnf_base.DnfBaseApiTest.test_do_transaction)
1: ----------------------------------------------------------------------
1: Traceback (most recent call last):
1:   File "/builddir/build/BUILD/dnf-4.19.0-0.20240130004727.4.18.2+6.g0e400470/tests/api/test_dnf_base.py", line 174, in test_do_transaction
1:     self.base.do_transaction(display=None)
1:   File "/builddir/build/BUILD/dnf-4.19.0-0.20240130004727.4.18.2+6.g0e400470/dnf/base.py", line 953, in do_transaction
1:     self._moduleContainer.save()
1:   File "/usr/lib64/python3.12/site-packages/libdnf/module.py", line 1241, in save
1:     return _module.ModulePackageContainer_save(self)
1:            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1: libdnf._error.Error: Failed to create directory "/etc/dnf/modules.d": 13 - Permission denied# Please enter the commit message for your changes. Lines starting

It is also failing in our nightlies.

tests/api/test_dnf_base.py

@@ -28,6 +28,7 @@ class DnfBaseApiTest(TestCase):
  def setUp(self):
  self.base = dnf.Base(dnf.conf.Conf())
  self.base.conf.persistdir = "/tmp/tests"
+ self.base.conf.installroot = "/tmp/dnf-test-installroot/"
 
  def tearDown(self):
  self.base.close()