[dbus] Enable default DBUS_SESSION_SOCKET_DIR when cross-compiling

[WangWeiLin-MV]

Fix #40031.

Allow setting default DBUS_SESSION_SOCKET_DIR, default /tmp.

Checklist

• [x] Changes comply with the maintainer guide.
• [ ] ~SHA512s are updated for each updated download.~
• [ ] ~The "supports" clause reflects platforms that may be fixed by this new version.~
• [ ] ~Any fixed CI baseline entries are removed from that file.~
• [x] Any patches that are no longer applied are deleted from the port's directory.
• [x] The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
• [x] Only one version is added to each modified port's versions file.

Test

The port cross-compile installation tests pass with the following triplets:

• x64-osx (on arm64-osx)