kbuild: pacman-pkg: hardcode module installation path

'make pacman-pkg' for architectures with device tree support (i.e., arm,
arm64, etc.) shows logs like follows:

  Installing dtbs...
    INSTALL /home/masahiro/linux/pacman/linux-upstream/pkg/linux-upstream/usr//lib/modules/6.14.0-rc6+/dtb/actions/s700-cubieboard7.dtb
    INSTALL /home/masahiro/linux/pacman/linux-upstream/pkg/linux-upstream/usr//lib/modules/6.14.0-rc6+/dtb/actions/s900-bubblegum-96.dtb
    INSTALL /home/masahiro/linux/pacman/linux-upstream/pkg/linux-upstream/usr//lib/modules/6.14.0-rc6+/dtb/airoha/en7581-evb.dtb
      ...

The double slashes ('//') between 'usr' and 'lib' are somewhat ugly.

Let's hardcode the module installation path because the package contents
should remain unaffected even if ${MODLIB} is overridden. Please note that
scripts/packages/{builddeb,kernel.spec} also hardcode the module
installation path.

With this change, the log will look better, as follows:

  Installing dtbs...
    INSTALL /home/masahiro/linux/pacman/linux-upstream/pkg/linux-upstream/usr/lib/modules/6.14.0-rc6+/dtb/actions/s700-cubieboard7.dtb
    INSTALL /home/masahiro/linux/pacman/linux-upstream/pkg/linux-upstream/usr/lib/modules/6.14.0-rc6+/dtb/actions/s900-bubblegum-96.dtb
    INSTALL /home/masahiro/linux/pacman/linux-upstream/pkg/linux-upstream/usr/lib/modules/6.14.0-rc6+/dtb/airoha/en7581-evb.dtb
      ...

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>