mkarchiso: update memtest86+ license file location

The licenses package does not ship
`/usr/share/licenses/common/GPL2/license.txt` anymore, which results in:

    /usr/share/licenses/common/GPL2/license.txt': No such file or directory

Use its replacement, `/usr/share/licenses/spdx/GPL-2.0-only.txt` instead.

Fixes https://gitlab.archlinux.org/archlinux/archiso/-/issues/225

CHANGELOG.rst

@@ -20,6 +20,8 @@ Deprecated
 Fixed
 -----
 
+- Update the location where ``mkarchiso`` looks for the memtest86+ license file.
+
 Removed
 -------
 

archiso/mkarchiso

@@ -482,7 +482,7 @@ _make_bootmode_bios.syslinux.mbr() {
         install -d -m 0755 -- "${isofs_dir}/boot/memtest86+/"
         # rename for PXE: https://wiki.archlinux.org/title/Syslinux#Using_memtest
         install -m 0644 -- "${pacstrap_dir}/boot/memtest86+/memtest.bin" "${isofs_dir}/boot/memtest86+/memtest"
-        install -m 0644 -- "${pacstrap_dir}/usr/share/licenses/common/GPL2/license.txt" "${isofs_dir}/boot/memtest86+/"
+        install -m 0644 -- "${pacstrap_dir}/usr/share/licenses/spdx/GPL-2.0-only.txt" "${isofs_dir}/boot/memtest86+/LICENSE"
     fi
     _msg_info "Done! SYSLINUX set up for BIOS booting from a disk successfully."
 }
@@ -796,7 +796,7 @@ _make_bootmode_uefi-x64.grub.esp() {
     if [[ -e "${pacstrap_dir}/boot/memtest86+/memtest.efi" ]]; then
         install -d -m 0755 -- "${isofs_dir}/boot/memtest86+/"
         install -m 0644 -- "${pacstrap_dir}/boot/memtest86+/memtest.efi" "${isofs_dir}/boot/memtest86+/memtest.efi"
-        install -m 0644 -- "${pacstrap_dir}/usr/share/licenses/common/GPL2/license.txt" "${isofs_dir}/boot/memtest86+/"
+        install -m 0644 -- "${pacstrap_dir}/usr/share/licenses/spdx/GPL-2.0-only.txt" "${isofs_dir}/boot/memtest86+/LICENSE"
     fi
 
     _msg_info "Done! GRUB set up for UEFI booting successfully."