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>

default.nix

@@ -15,19 +15,6 @@ stdenv.mkDerivation rec {
       libsodium libscrypt
     ];
 
-  patchPhase = ''
-    # ensure the mkfs and fsck scripts, which are just wrappers around
-    # 'bcachefs', are patched to refer to the right location inside the
-    # nix store. (you wouldn't expect built tools to call random outside
-    # utilities, in general, but the exact tools they were built with.)
-    #
-    # TODO FIXME: it would be better to fix this in the 'install' target,
-    # however, so this works with any bog-standard installation
-
-    substituteInPlace fsck.bcachefs --replace bcachefs $out/bin/bcachefs
-    substituteInPlace mkfs.bcachefs --replace bcachefs $out/bin/bcachefs
-  '';
-
   enableParallelBuilding = true;
   makeFlags =
     [ "PREFIX=$(out)"

fsck.bcachefs

@@ -1,3 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env bash
 
-exec bcachefs fsck "$@"
+SDIR="$(dirname "$(readlink -f "$0")")"
+exec "$SDIR/bcachefs" fsck "$@"

mkfs.bcachefs

@@ -1,3 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env bash
 
-exec bcachefs format "$@"
+SDIR="$(dirname "$(readlink -f "$0")")"
+exec "$SDIR/bcachefs" format "$@"