debian/rules: work around lrelease failure when building under user-mode emulation.
In user-mode emulation, lrelease wrongly detects a cross-compiling situation and fails to load the default mkspec for some reason. Debugging this is incredibly frustrating, especially since strace and gdb do not work correctly in user-mode emulation.
Fortunately, we can work around this issue by specifying QMAKESPEC directly, ironically with values queried from qmake.
We will do that for Debian 12+ only for now. Note that this is not an issue with full-machine emulation, since the inner system is unsuspecting to the actual bare-metal host.
N.B.: I need this ASAP, so will merge this as soon as the test build succeeds.