Make cachedir relative to installroot

Like `logdir`, the `cachedir` configuration option should be treated as
relative to the installroot, unless it is specified on the command line
via --setopt=cachedir=/path/to/cachedir. This is the behavior of
the cachedir with DNF 4.

Currently, the host cache directory is used by default, conflicting with
what our docs say (man 7 dnf5-installroot):
    "cachedir, log files, releasever, and gpgkey are taken from or stored
    in the installroot."
After this change, the default cachedir is
$installroot/var/cache/libdnf5, which is consistent with the docs.

For https://bugzilla.redhat.com/show_bug.cgi?id=2256945

doc/misc/installroot.7.rst

@@ -43,6 +43,7 @@ will be used.
 Note: When a path is specified within a command line argument
 (``--config=CONFIG_FILE_PATH`` in case of `configuration` file,
 ``--setopt=reposdir=/path/to/repodir`` for `reposdir`,
+``--setopt=cachedir=/path/to/cachedir`` for `cachedir`,
 ``--setopt=logdir=/path/to/logdir`` for `logdir`, or
 ``--setopt=varsdir=/paths/to/varsdir`` for `vars`), then this path is always
 relative to the host with no exceptions. `pluginpath` and `pluginconfpath` are

libdnf5/base/base.cpp

@@ -165,12 +165,17 @@ void Base::setup() {
             vars_installroot = config.get_installroot_option().get_value();
         }
     }
-    // Unless the logdir is specified on the command line, logdir should be
-    // relative to the installroot
+    // Unless the cachedir or logdir are specified on the command line, they
+    // should be relative to the installroot
     if (config.get_logdir_option().get_priority() < Option::Priority::COMMANDLINE) {
         const std::filesystem::path logdir_path{config.get_logdir_option().get_value()};
-        const auto full_path = (installroot_path / logdir_path.relative_path()).string();
-        config.get_logdir_option().set(Option::Priority::INSTALLROOT, full_path);
+        const auto full_path = installroot_path / logdir_path.relative_path();
+        config.get_logdir_option().set(Option::Priority::INSTALLROOT, full_path.string());
+    }
+    if (config.get_cachedir_option().get_priority() < Option::Priority::COMMANDLINE) {
+        const std::filesystem::path cachedir_path{config.get_cachedir_option().get_value()};
+        const auto full_path = installroot_path / cachedir_path.relative_path();
+        config.get_cachedir_option().set(Option::Priority::INSTALLROOT, full_path.string());
     }
 
     load_plugins();