fs/ubifs: work around fakeroot / sed -i issue on systems with SELinux enabled

Fixes #9386

GNU sed -i misbehaves on systems with SELinux enabled, as it tries to copy
to the SELinux security context (xattr) from the source file to to the new
destination file, which fails under fakeroot and leaves the file with 000
permissions, causing ubinize to fail when it cannot read to configuration
file.

So as a workaround, combine the install and tweak steps in a single
sed with a redirect to the destination file instead.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>