make: correct path to 'bcachefs' in mkfs/fsck scripts

make: correct path to 'bcachefs' in mkfs/fsck scripts

The fsck and bcachefs scripts simply worked by running 'exec bcachefs
...', but this executes whatever is out of `$PATH`, which is likely not
what a user expected when working inside the bcachefs-tools tree.

This replaces the local uses of 'exec bcachefs' to use bash's
readlink/dirname builtins in order to find the location of the currently
executing wrapper, and execute the 'bcachefs' tool from there.

This allows executing these wrappers from anywhere, provided 'bcachefs'
is just right next to them, with the correct semantics.

As a result, this also allows removing a tiny hack from the Nix
expressions, allowing the included default.nix to use completely
standard mkDerivation builders, with no extra patch/fixup phases.

Signed-off-by: Austin Seipp <aseipp@pobox.com>