rpm: add a disableUnshare option

Add an option to disable the unshare RPM plugin. Enabling it hinders execution within the Nix sandbox. While plain RPM invocations can use `--noplugins` and friends, many consumers like `tdnf` do not expose such a knob for enabling / disabling RPM options. Thus, add an easy interface to disable the plugin via Nix.

pkgs/tools/package-management/rpm/default.nix

@@ -29,6 +29,10 @@
   bubblewrap,
   autoconf,
   gnupg,
+
+  # Disable the unshare RPM plugin, which can be useful if
+  # RPM is ran within the Nix sandbox.
+  disableUnshare ? true,
 }:
 
 stdenv.mkDerivation rec {
@@ -112,6 +116,9 @@ stdenv.mkDerivation rec {
     ++ lib.optionals stdenv.hostPlatform.isDarwin [
       "-DWITH_LIBELF=OFF"
       "-DWITH_LIBDW=OFF"
+    ]
+    ++ lib.optionals disableUnshare [
+      "-DHAVE_UNSHARE=OFF"
     ];
 
   # rpm/rpmlib.h includes popt.h, and then the pkg-config file mentions these as linkage requirements